Update readme, add some tests

Signed-off-by: Gabriel Adrian Samfira <gsamfira@cloudbasesolutions.com>

README.md

@@ -17,9 +17,16 @@ git clone https://github.com/cloudbase/garm
 cd garm
 go install ./...
 ```
-
 You should now have both ```garm``` and ```garm-cli``` in your ```$GOPATH/bin``` folder.
 
+If you have docker/podman installed, you can also build statically linked binaries by running:
+
+```bash
+make
+```
+
+The ```garm``` and ```garm-cli``` binaries will be built and copied to the ```bin/``` folder in your current working directory.
+
 ## Install the service
 
 Add a new system user:
@@ -31,12 +38,20 @@ useradd --shell /usr/bin/false \
     --no-create-home garm
 ```
 
-Copy the binary from your ```$GOPATH``` to somewhere in the system ```$PATH```:
+The ```lxd``` group is only needed if you have a local LXD install and want to connect to the unix socket to use it. If you're connecting to a remote LXD server over TCP, you can skip adding the ```garm``` user to the ```lxd``` group.
+
+Copy the binary to somewhere in the system ```$PATH```:
 
 ```bash
 sudo cp $(go env GOPATH)/bin/garm /usr/local/bin/garm
 ```
 
+Or if you built garm using ```make```:
+
+```bash
+sudo cp ./bin/garm /usr/local/bin/garm
+```
+
 Create the config folder:
 
 ```bash

config/config.go

@@ -165,6 +165,12 @@ func (d *Default) Validate() error {
 		return fmt.Errorf("missing callback_url")
 	}
 
+	if d.ConfigDir != "" {
+		if _, err := os.Stat(d.ConfigDir); err != nil {
+			return errors.Wrap(err, "accessing config dir")
+		}
+	}
+
 	return nil
 }
 

config/config_test.go

@@ -0,0 +1 @@
+package config

config/external_test.go

@@ -1,6 +1,7 @@
 package config
 
 import (
+	"fmt"
 	"io/ioutil"
 	"os"
 	"path/filepath"
@@ -72,6 +73,14 @@ func TestExternal(t *testing.T) {
 			},
 			errString: "path to provider dir must be absolute",
 		},
+		{
+			name: "Provider executable not found",
+			cfg: External{
+				ConfigFile:  "",
+				ProviderDir: "/tmp",
+			},
+			errString: "checking provider executable: stat /tmp/garm-external-provider: no such file or directory",
+		},
 	}
 
 	for _, tc := range tests {
@@ -86,3 +95,16 @@ func TestExternal(t *testing.T) {
 		})
 	}
 }
+
+func TestProviderExecutableIsExecutable(t *testing.T) {
+	cfg := getDefaultExternalConfig(t)
+
+	execPath, err := cfg.ExecutablePath()
+	assert.Nil(t, err)
+	err = os.Chmod(execPath, 0o644)
+	assert.Nil(t, err)
+
+	err = cfg.Validate()
+	assert.NotNil(t, err)
+	assert.EqualError(t, err, fmt.Sprintf("external provider binary %s is not executable", execPath))
+}