---
title: "Docker"
linkTitle: "Docker"
weight: 10
description: >
  Container runtime for running containerized applications
---

## Overview

Docker is a container platform that packages applications and dependencies into standardized units called containers. In the Edge Developer Platform, Docker serves three primary functions: powering local development environments through Docker Desktop, building container images in CI/CD pipelines, and providing Docker-in-Docker (DinD) execution environments for Forgejo Actions runners.

Docker provides a consistent runtime environment across development, testing, and production, ensuring applications behave identically regardless of the underlying infrastructure.

## Key Features

* **Container Runtime**: Execute isolated application containers with process, network, and filesystem isolation
* **Image Building**: Create container images using Dockerfile specifications and layer caching
* **Docker-in-Docker**: Nested Docker execution for CI/CD runners and containerized build environments
* **Multi-stage Builds**: Optimize image size through efficient build processes
* **Volume Management**: Persistent data storage with bind mounts and named volumes
* **Network Isolation**: Software-defined networking with bridge, host, and overlay networks
* **Resource Control**: CPU, memory, and I/O limits through Linux cgroups

## Purpose in EDP

Docker plays several critical roles in the Edge Developer Platform:

**Local Development**: Docker Desktop provides developers with containerized development environments, enabling consistent tooling and dependencies across different machines. Developers can run entire application stacks locally using docker-compose configurations.

**Image Building**: CI/CD pipelines use Docker to build container images from source code. The `docker build` command transforms Dockerfiles into layered images that can be deployed to Kubernetes or other container orchestration platforms.

**CI/CD Execution**: Forgejo Actions runners use Docker-in-Docker to execute workflow steps in isolated containers. Each job runs in a fresh container environment, ensuring clean state and reproducible builds.

## Repository

**Code**: Docker is an open-source project

**Documentation**:
* [Docker Official Documentation](https://docs.docker.com/)
* [Containerd Documentation](https://containerd.io/docs/)
* [OCI Runtime Specification](https://github.com/opencontainers/runtime-spec)
* [OverlayFS Documentation](https://www.kernel.org/doc/html/latest/filesystems/overlayfs.html)

## Getting Started

### Prerequisites

* Linux kernel 3.10+ with namespace and cgroup support
* 64-bit processor architecture (x86_64, ARM64)
* At least 4GB RAM for Docker Desktop
* 20GB available disk space for images and containers

### Quick Start

**Install Docker on Linux**:

```bash
# Install Docker using official script
curl -fsSL https://get.docker.com -o get-docker.sh
sudo sh get-docker.sh

# Add user to docker group (optional, allows running docker without sudo)
sudo usermod -aG docker $USER

# Start Docker daemon
sudo systemctl start docker
sudo systemctl enable docker
```

**Install Docker Desktop** (macOS/Windows):

1. Download Docker Desktop from [docker.com](https://www.docker.com/products/docker-desktop)
2. Run the installer
3. Launch Docker Desktop from Applications
4. Verify installation in system tray/menu bar

### Verification

Verify Docker installation:

```bash
# Check Docker version
docker --version

# Verify daemon is running
docker info

# Run test container
docker run hello-world

# Check running containers
docker ps

# View all containers (including stopped)
docker ps -a

# List downloaded images
docker images
```

## Usage Examples

### Running Containers

**Basic Container Execution**:

```bash
# Run container in foreground
docker run ubuntu:22.04 echo "Hello from container"

# Run container in background (detached)
docker run -d --name nginx-server nginx:latest

# Run interactive container with shell
docker run -it ubuntu:22.04 /bin/bash

# Run container with port mapping
docker run -d -p 8080:80 nginx:latest

# Run container with volume mount
docker run -d -v /host/data:/container/data ubuntu:22.04

# Run container with environment variables
docker run -d -e POSTGRES_PASSWORD=secret postgres:15

# Run container with resource limits
docker run -d --memory=512m --cpus=1.5 nginx:latest
```

**Container Management**:

```bash
# Stop container
docker stop nginx-server

# Start stopped container
docker start nginx-server

# Restart container
docker restart nginx-server

# View container logs
docker logs nginx-server
docker logs -f nginx-server  # Follow logs

# Execute command in running container
docker exec -it nginx-server /bin/bash

# Copy files between host and container
docker cp myfile.txt nginx-server:/tmp/
docker cp nginx-server:/tmp/myfile.txt ./

# Inspect container details
docker inspect nginx-server

# View container resource usage
docker stats nginx-server

# Remove container
docker rm nginx-server
docker rm -f nginx-server  # Force remove running container
```

### Building Images

**Simple Dockerfile**:

```dockerfile
# Dockerfile
FROM ubuntu:22.04

# Install dependencies
RUN apt-get update && apt-get install -y \
    curl \
    vim \
    && rm -rf /var/lib/apt/lists/*

# Set working directory
WORKDIR /app

# Copy application files
COPY . /app

# Set environment variable
ENV APP_ENV=production

# Expose port
EXPOSE 8080

# Define entrypoint
CMD ["./start.sh"]
```

**Build and Tag Image**:

```bash
# Build image from Dockerfile
docker build -t myapp:1.0 .

# Build with build arguments
docker build --build-arg VERSION=1.0 -t myapp:1.0 .

# Build without cache
docker build --no-cache -t myapp:1.0 .

# Tag image for registry
docker tag myapp:1.0 registry.example.com/myapp:1.0

# Push to registry
docker push registry.example.com/myapp:1.0

# Pull from registry
docker pull registry.example.com/myapp:1.0
```

**Multi-stage Build**:

```dockerfile
# Dockerfile with multi-stage build
FROM golang:1.21 AS builder

WORKDIR /src
COPY go.mod go.sum ./
RUN go mod download

COPY . .
RUN CGO_ENABLED=0 go build -o /app/server

# Final stage
FROM alpine:3.19

RUN apk --no-cache add ca-certificates
COPY --from=builder /app/server /usr/local/bin/server

USER nobody
EXPOSE 8080
CMD ["server"]
```

Multi-stage builds reduce final image size by excluding build tools and intermediate artifacts.

### Docker-in-Docker for CI/CD

**DinD Container for Building Images**:

```bash
# Start DinD daemon
docker run -d \
  --name dind \
  --privileged \
  -e DOCKER_TLS_CERTDIR=/certs \
  -v docker-certs:/certs \
  docker:28.0.4-dind

# Run build container connected to DinD
docker run --rm \
  -e DOCKER_HOST=tcp://dind:2376 \
  -e DOCKER_TLS_VERIFY=1 \
  -e DOCKER_CERT_PATH=/certs/client \
  -v docker-certs:/certs:ro \
  -v $(pwd):/workspace \
  -w /workspace \
  --link dind:dind \
  docker:28.0.4-cli \
  docker build -t myapp:latest .
```

**Kubernetes DinD Sidecar (Forgejo Runner Pattern)**:

```yaml
apiVersion: v1
kind: Pod
metadata:
  name: forgejo-runner
spec:
  containers:
  - name: runner
    image: code.forgejo.org/forgejo/runner:6.4.0
    env:
    - name: DOCKER_HOST
      value: tcp://localhost:2376
    - name: DOCKER_TLS_VERIFY
      value: "1"
    - name: DOCKER_CERT_PATH
      value: /certs/client
    volumeMounts:
    - name: docker-certs
      mountPath: /certs
      readOnly: true
    - name: runner-data
      mountPath: /data

  - name: dind
    image: docker:28.0.4-dind
    securityContext:
      privileged: true
    env:
    - name: DOCKER_TLS_CERTDIR
      value: /certs
    volumeMounts:
    - name: docker-certs
      mountPath: /certs
    - name: docker-storage
      mountPath: /var/lib/docker

  volumes:
  - name: docker-certs
    emptyDir: {}
  - name: runner-data
    emptyDir: {}
  - name: docker-storage
    emptyDir: {}
```

This configuration runs a Forgejo Actions runner with a DinD sidecar, enabling containerized builds within Kubernetes pods.

### Local Development with Docker Compose

**Docker Compose Configuration**:

```yaml
# docker-compose.yml
version: '3.8'

services:
  app:
    build: .
    ports:
      - "8080:8080"
    environment:
      - DATABASE_URL=postgresql://user:pass@db:5432/appdb
    depends_on:
      - db
      - redis
    volumes:
      - ./src:/app/src

  db:
    image: postgres:15
    environment:
      - POSTGRES_USER=user
      - POSTGRES_PASSWORD=pass
      - POSTGRES_DB=appdb
    volumes:
      - postgres-data:/var/lib/postgresql/data

  redis:
    image: redis:7-alpine
    ports:
      - "6379:6379"

volumes:
  postgres-data:
```

**Compose Commands**:

```bash
# Start all services
docker-compose up -d

# View logs
docker-compose logs -f

# Stop services
docker-compose down

# Rebuild and restart
docker-compose up -d --build

# Run command in service
docker-compose exec app /bin/bash

# Scale service
docker-compose up -d --scale app=3
```

### Image Management

**Image Operations**:

```bash
# List images
docker images

# Remove image
docker rmi myapp:1.0

# Remove unused images
docker image prune

# Remove all unused images
docker image prune -a

# Inspect image layers
docker history myapp:1.0
```

**Registry Operations**:

```bash
# Login to registry
docker login registry.example.com

# Login with credentials
echo $PASSWORD | docker login -u $USERNAME --password-stdin registry.example.com

# Push image
docker push registry.example.com/myapp:1.0

# Pull image
docker pull registry.example.com/myapp:1.0

# Search registry
docker search nginx
```

## Architecture

### Docker Architecture Overview

Docker follows a client-server architecture with several key components:

**Docker Client**: Command-line tool (docker CLI) that sends API requests to the Docker daemon. Developers interact with Docker through client commands like `docker run`, `docker build`, and `docker ps`.

**Docker Daemon (dockerd)**: Background service that manages containers, images, networks, and volumes. The daemon listens for Docker API requests and coordinates with lower-level runtime components.

**Containerd**: High-level container runtime that manages container lifecycle operations. Containerd handles image transfer and storage, container execution supervision, and low-level storage and network attachments.

**runc**: Low-level OCI (Open Container Initiative) runtime that creates and runs containers. Runc interfaces directly with Linux kernel features to spawn and manage container processes.

### Container Runtime Layers

Docker's layered architecture separates concerns across multiple components:

```
┌─────────────────────────────────┐
│      Docker Client (CLI)        │
│   docker run, build, push       │
└────────────┬────────────────────┘
             │ Docker API
┌────────────▼────────────────────┐
│      Docker Daemon (dockerd)    │
│   API, Image management, Auth   │
└────────────┬────────────────────┘
             │ Container API
┌────────────▼────────────────────┐
│         Containerd              │
│  Lifecycle, Image pull/push     │
└────────────┬────────────────────┘
             │ OCI Runtime API
┌────────────▼────────────────────┐
│           runc                  │
│  Namespace, cgroup creation     │
└────────────┬────────────────────┘
             │ System Calls
┌────────────▼────────────────────┐
│       Linux Kernel              │
│  namespaces, cgroups, seccomp   │
└─────────────────────────────────┘
```

**Docker Daemon Layer**: Handles high-level operations like image building, authentication, and API endpoint management. The daemon translates user commands into lower-level runtime operations.

**Containerd Layer**: Manages container lifecycle independent of Docker-specific features. Containerd can be used directly by Kubernetes and other orchestrators, providing a standard container runtime interface.

**runc Layer**: Implements the OCI runtime specification, creating container processes with Linux kernel isolation features. Runc configures namespaces, cgroups, and security policies before executing container entrypoints.

### Linux Kernel Features

Docker leverages several Linux kernel capabilities for container isolation:

**Namespaces**: Provide process isolation by creating separate views of system resources:
- **PID namespace**: Isolates process IDs so containers see only their own processes
- **Network namespace**: Provides separate network stacks with unique IP addresses and routing tables
- **Mount namespace**: Isolates filesystem mount points so containers have independent filesystem views
- **UTS namespace**: Separates hostname and domain name
- **IPC namespace**: Isolates inter-process communication resources like shared memory
- **User namespace**: Maps container user IDs to different host user IDs for privilege separation

**Control Groups (cgroups)**: Limit and account for resource usage:
- CPU allocation and throttling
- Memory limits and swap control
- Block I/O bandwidth limits
- Network bandwidth control (via tc integration)

**Capabilities**: Fine-grained privilege control that breaks down root privileges into discrete capabilities. Containers run with reduced capability sets, dropping dangerous privileges like CAP_SYS_ADMIN.

**Seccomp**: Filters system calls that containerized processes can make, reducing the kernel attack surface. Docker applies a default seccomp profile blocking ~44 dangerous syscalls.

**AppArmor/SELinux**: Mandatory access control systems that enforce security policies on container processes, restricting file access and operations.

### Image Storage and OverlayFS

Docker uses storage drivers to manage image layers and container filesystems. The preferred storage driver is overlay2, which uses OverlayFS:

**Image Layers**: Docker images consist of read-only layers stacked on top of each other. Each Dockerfile instruction creates a new layer. Layers are shared between images, reducing storage requirements.

**Copy-on-Write (CoW)**: When a container modifies a file from an image layer, OverlayFS copies the file to the container's writable layer. The original image layer remains unchanged, enabling efficient image reuse.

**OverlayFS Structure**:
```
Container Writable Layer (upperdir)
          │
          ├─ Modified files
          │
Image Layers (lowerdir)
          │
          ├─ Layer 3 (READ-ONLY)
          ├─ Layer 2 (READ-ONLY)
          └─ Layer 1 (READ-ONLY)
          │
    Merged View (merged)
          │
          └─ Union of all layers
```

**LowerDir**: Read-only image layers containing base filesystem and application files

**UpperDir**: Writable container layer where all changes are stored

**MergedDir**: Union mount that presents a unified view of all layers to the container

**WorkDir**: Internal working directory used by OverlayFS for atomic operations

When a container reads a file, OverlayFS serves it from the topmost layer where it exists. Writes always go to the upperdir, leaving image layers immutable.

### Networking Architecture

Docker provides several networking modes:

**Bridge Network**: Default network mode that creates a virtual bridge (docker0) on the host. Containers connect to the bridge and receive private IP addresses. Network Address Translation (NAT) enables outbound connectivity.

**Host Network**: Container shares the host's network namespace, using the host's IP address directly. Offers maximum network performance but reduces isolation.

**Overlay Network**: Multi-host networking for container communication across different Docker hosts. Used by Docker Swarm and can be integrated with Kubernetes.

**None**: Disables networking for maximum isolation.

### Docker-in-Docker (DinD) Architecture

Docker-in-Docker runs a nested Docker daemon inside a container. This is used in CI/CD runners to provide isolated build environments:

**Privileged Container**: DinD requires privileged mode to mount filesystems and create namespaces within the container. The `--privileged` flag grants extended capabilities.

**Separate Daemon**: A complete Docker daemon runs inside the container, managing its own containers, images, and networks independently of the host daemon.

**Certificate Management**: DinD uses mutual TLS authentication between the inner client and daemon. Certificates are shared through volumes mounted at `/certs`.

**Storage Driver**: The inner Docker daemon typically uses vfs or overlay2 storage driver. VFS provides maximum compatibility but larger storage overhead.

**Use in Forgejo Runners**:
```yaml
containers:
  - name: runner
    image: code.forgejo.org/forgejo/runner:6.4.0
    env:
      - name: DOCKER_HOST
        value: tcp://localhost:2376
      - name: DOCKER_TLS_VERIFY
        value: "1"

  - name: dind
    image: docker:28.0.4-dind
    securityContext:
      privileged: true
    volumeMounts:
      - name: docker-certs
        mountPath: /certs
```

The runner container connects to the DinD container via TCP, allowing workflow steps to execute docker commands for building and testing.

## Configuration

### Docker Daemon Configuration

The Docker daemon reads configuration from `/etc/docker/daemon.json`:

```json
{
  "log-driver": "json-file",
  "log-opts": {
    "max-size": "10m",
    "max-file": "3"
  },
  "storage-driver": "overlay2",
  "storage-opts": [
    "overlay2.override_kernel_check=true"
  ],
  "default-address-pools": [
    {
      "base": "172.17.0.0/16",
      "size": 24
    }
  ],
  "dns": ["8.8.8.8", "8.8.4.4"],
  "insecure-registries": [],
  "registry-mirrors": [],
  "features": {
    "buildkit": true
  },
  "max-concurrent-downloads": 10,
  "max-concurrent-uploads": 5
}
```

**Key Configuration Options**:

* `log-driver`: Logging mechanism (json-file, syslog, journald, etc.)
* `storage-driver`: Filesystem driver (overlay2, devicemapper, btrfs, zfs)
* `insecure-registries`: Registries that don't require HTTPS
* `registry-mirrors`: Mirror registries for faster pulls
* `buildkit`: Enable BuildKit for improved build performance

**Apply Configuration Changes**:

```bash
# Restart Docker daemon
sudo systemctl restart docker

# Verify configuration
docker info
```

### Docker-in-Docker Configuration

**Environment Variables**:

* `DOCKER_TLS_CERTDIR`: Directory for TLS certificates (typically `/certs`)
* `DOCKER_HOST`: Docker daemon address (e.g., `tcp://localhost:2376`)
* `DOCKER_TLS_VERIFY`: Enable TLS verification (`1` or `0`)
* `DOCKER_CERT_PATH`: Path to client certificates

**DinD Security Considerations**:

DinD requires privileged mode, which grants extended capabilities. Use DinD only in trusted environments:
- CI/CD runners in isolated namespaces
- Development environments
- Build systems with network isolation

Avoid using DinD for untrusted workloads or multi-tenant environments.

## Integration Points

* **Kubernetes**: Can use Docker (via dockershim, deprecated) or containerd directly as container runtime
* **Forgejo Actions**: Uses Docker-in-Docker for isolated build execution in CI/CD pipelines
* **Container Registries**: Pushes and pulls images to/from OCI-compliant registries
* **Development Environments**: Docker Desktop provides local container runtime for development
* **Image Scanning Tools**: Integrates with security scanners like Trivy and Clair
* **Monitoring Systems**: Exports metrics via Prometheus exporters and logging drivers

## Troubleshooting

### Docker Daemon Won't Start

**Problem**: Docker daemon fails to start or crashes immediately

**Solution**:
1. Check daemon logs:
   ```bash
   sudo journalctl -u docker.service
   sudo cat /var/log/docker.log
   ```

2. Verify kernel support:
   ```bash
   docker info | grep -i kernel
   grep CONFIG_NAMESPACES /boot/config-$(uname -r)
   ```

3. Test daemon in debug mode:
   ```bash
   sudo dockerd --debug
   ```

4. Check for port conflicts:
   ```bash
   sudo netstat -tulpn | grep docker
   ```

### Container Cannot Connect to Network

**Problem**: Container has no network connectivity or DNS resolution fails

**Solution**:
1. Check container network mode:
   ```bash
   docker inspect container-name | grep -i network
   ```

2. Verify DNS configuration:
   ```bash
   docker exec container-name cat /etc/resolv.conf
   docker exec container-name ping 8.8.8.8
   docker exec container-name ping google.com
   ```

3. Check firewall rules:
   ```bash
   sudo iptables -L -n | grep DOCKER
   ```

4. Restart Docker network:
   ```bash
   sudo systemctl restart docker
   ```

5. Recreate default bridge:
   ```bash
   docker network rm bridge
   sudo systemctl restart docker
   ```

### Out of Disk Space

**Problem**: Docker runs out of disk space for images or containers

**Solution**:
1. Check disk usage:
   ```bash
   docker system df
   docker system df -v  # Verbose output
   ```

2. Remove unused containers:
   ```bash
   docker container prune
   ```

3. Remove unused images:
   ```bash
   docker image prune -a
   ```

4. Remove unused volumes:
   ```bash
   docker volume prune
   ```

5. Complete cleanup:
   ```bash
   docker system prune -a --volumes
   ```

6. Configure log rotation in `/etc/docker/daemon.json`:
   ```json
   {
     "log-opts": {
       "max-size": "10m",
       "max-file": "3"
     }
   }
   ```

### Docker Build Fails

**Problem**: Image build fails with errors or hangs

**Solution**:
1. Build with verbose output:
   ```bash
   docker build --progress=plain --no-cache -t myapp .
   ```

2. Check Dockerfile syntax:
   ```bash
   docker build --check -t myapp .
   ```

3. Verify base image exists:
   ```bash
   docker pull ubuntu:22.04
   ```

4. Increase build memory (Docker Desktop):
   - Open Docker Desktop settings
   - Increase memory allocation to 4GB+

5. Check build context size:
   ```bash
   docker build --progress=plain -t myapp . 2>&1 | grep "transferring context"
   ```

6. Use `.dockerignore` to exclude large files:
   ```
   # .dockerignore
   node_modules
   .git
   *.log
   ```

### Docker-in-Docker Container Cannot Build Images

**Problem**: DinD container fails to build images or start daemon

**Solution**:
1. Verify privileged mode:
   ```bash
   docker inspect dind | grep -i privileged
   ```

2. Check DinD daemon logs:
   ```bash
   docker logs dind
   ```

3. Verify certificate volumes:
   ```bash
   docker exec dind ls -la /certs
   ```

4. Test Docker client connection:
   ```bash
   docker run --rm \
     -e DOCKER_HOST=tcp://dind:2376 \
     -e DOCKER_TLS_VERIFY=1 \
     -e DOCKER_CERT_PATH=/certs/client \
     -v docker-certs:/certs:ro \
     --link dind:dind \
     docker:28.0.4-cli \
     docker info
   ```

5. Check storage driver:
   ```bash
   docker exec dind docker info | grep "Storage Driver"
   ```

### Permission Denied When Running Docker

**Problem**: User cannot run Docker commands without sudo

**Solution**:
1. Add user to docker group:
   ```bash
   sudo usermod -aG docker $USER
   ```

2. Log out and back in to apply group changes

3. Verify group membership:
   ```bash
   groups $USER
   ```

4. Test Docker access:
   ```bash
   docker ps
   ```

5. If issue persists, check socket permissions:
   ```bash
   sudo chmod 666 /var/run/docker.sock
   ```

Note: Adding users to the docker group grants root-equivalent privileges. Only add trusted users.

## Additional Resources

* [Docker Documentation](https://docs.docker.com/)
* [Docker Hub](https://hub.docker.com/)
* [Dockerfile Best Practices](https://docs.docker.com/develop/dev-best-practices/)
* [Containerd Documentation](https://containerd.io/)
* [OCI Specifications](https://opencontainers.org/)
* [Docker Security Best Practices](https://docs.docker.com/engine/security/)
* [Kernel Namespace Documentation](https://man7.org/linux/man-pages/man7/namespaces.7.html)
* [Control Groups (cgroups) Documentation](https://www.kernel.org/doc/html/latest/admin-guide/cgroup-v2.html)