feat(docker): multi-stage build for collector and receiver

Add multi-stage Dockerfile that can build both images:
- `docker build --target collector` for the collector
- `docker build --target receiver` for the receiver (with CGO for SQLite)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Dockerfile

@@ -1,16 +1,37 @@
-FROM golang:1.25-alpine AS builder
+FROM golang:1.25-alpine AS builder-base
 
 WORKDIR /app
 
-COPY go.mod ./
+COPY go.mod go.sum ./
 RUN go mod download
 
 COPY . .
 
+# Collector build (no CGO needed)
+FROM builder-base AS builder-collector
+
 RUN CGO_ENABLED=0 GOOS=linux go build -ldflags="-s -w" -o /resource-collector ./cmd/collector
 
-FROM alpine:3.19
+# Receiver build (CGO needed for SQLite)
+FROM builder-base AS builder-receiver
 
-COPY --from=builder /resource-collector /usr/local/bin/resource-collector
+RUN apk add --no-cache gcc musl-dev
+RUN CGO_ENABLED=1 GOOS=linux go build -ldflags="-s -w" -o /metrics-receiver ./cmd/receiver
+
+# Collector image
+FROM alpine:3.19 AS collector
+
+COPY --from=builder-collector /resource-collector /usr/local/bin/resource-collector
 
 ENTRYPOINT ["/usr/local/bin/resource-collector"]
+
+# Receiver image
+FROM alpine:3.19 AS receiver
+
+RUN apk add --no-cache sqlite-libs
+
+COPY --from=builder-receiver /metrics-receiver /usr/local/bin/metrics-receiver
+
+EXPOSE 8080
+
+ENTRYPOINT ["/usr/local/bin/metrics-receiver"]