builder/docker: verify docker is available on path

builder/docker/builder.go

@@ -25,6 +25,11 @@ func (b *Builder) Prepare(raws ...interface{}) ([]string, error) {
 }
 
 func (b *Builder) Run(ui packer.Ui, hook packer.Hook, cache packer.Cache) (packer.Artifact, error) {
+	driver := &DockerDriver{Ui: ui}
+	if err := driver.Verify(); err != nil {
+		return nil, err
+	}
+
 	steps := []multistep.Step{
 		&StepTempDir{},
 		&StepPull{},
@@ -40,9 +45,7 @@ func (b *Builder) Run(ui packer.Ui, hook packer.Hook, cache packer.Cache) (packe
 	state.Put("ui", ui)
 
 	// Setup the driver that will talk to Docker
-	state.Put("driver", &DockerDriver{
+	state.Put("driver", driver)
-		Ui: ui,
-	})
 
 	// Run!
 	if b.config.PackerDebug {

builder/docker/driver.go

@@ -20,6 +20,9 @@ type Driver interface {
 
 	// StopContainer forcibly stops a container.
 	StopContainer(id string) error
+
+	// Verify verifies that the driver can run
+	Verify() error
 }
 
 // ContainerConfig is the configuration used to start a container.

builder/docker/driver_docker.go

@@ -82,3 +82,11 @@ func (d *DockerDriver) StartContainer(config *ContainerConfig) (string, error) {
 func (d *DockerDriver) StopContainer(id string) error {
 	return exec.Command("docker", "kill", id).Run()
 }
+
+func (d *DockerDriver) Verify() error {
+	if _, err := exec.LookPath("docker"); err != nil {
+		return err
+	}
+
+	return nil
+}

builder/docker/driver_mock.go

@@ -12,6 +12,7 @@ type MockDriver struct {
 	StartID      string
 	StartError   error
 	StopError    error
+	VerifyError  error
 
 	ExportCalled bool
 	ExportID     string
@@ -21,6 +22,7 @@ type MockDriver struct {
 	StartConfig  *ContainerConfig
 	StopCalled   bool
 	StopID       string
+	VerifyCalled bool
 }
 
 func (d *MockDriver) Export(id string, dst io.Writer) error {
@@ -54,3 +56,8 @@ func (d *MockDriver) StopContainer(id string) error {
 	d.StopID = id
 	return d.StopError
 }
+
+func (d *MockDriver) Verify() error {
+	d.VerifyCalled = true
+	return d.VerifyError
+}