feat: use CMD/ENTRYPOINT from source image by default

builder/docker/builder.go

@@ -89,6 +89,7 @@ func (b *Builder) Run(ctx context.Context, ui packersdk.Ui, hook packersdk.Hook)
 		log.Print("[DEBUG] Container will be discarded")
 	} else if b.config.Commit {
 		log.Print("[DEBUG] Container will be committed")
+		steps = append(steps, &StepSetDefaults{})
 		steps = append(steps, &StepCommit{
 			GeneratedData: generatedData,
 		})

builder/docker/driver_docker.go

@@ -214,6 +214,40 @@ func (d *DockerDriver) Digest(id string) (string, error) {
 	return strings.TrimSpace(stdout.String()), nil
 }
 
+func (d *DockerDriver) Cmd(id string) (string, error) {
+	var stderr, stdout bytes.Buffer
+	cmd := exec.Command(
+		"docker",
+		"inspect",
+		"--format",
+		"{{if .Config.Cmd}} {{json .Config.Cmd}} {{else}} [] {{end}}",
+		id)
+	cmd.Stdout = &stdout
+	cmd.Stderr = &stderr
+	if err := cmd.Run(); err != nil {
+		return "", fmt.Errorf("Error: %s\n\nStderr: %s", err, stderr.String())
+	}
+
+	return strings.TrimSpace(stdout.String()), nil
+}
+
+func (d *DockerDriver) Entrypoint(id string) (string, error) {
+	var stderr, stdout bytes.Buffer
+	cmd := exec.Command(
+		"docker",
+		"inspect",
+		"--format",
+		"{{if .Config.Entrypoint}} {{json .Config.Entrypoint}} {{else}} [] {{end}}",
+		id)
+	cmd.Stdout = &stdout
+	cmd.Stderr = &stderr
+	if err := cmd.Run(); err != nil {
+		return "", fmt.Errorf("Error: %s\n\nStderr: %s", err, stderr.String())
+	}
+
+	return strings.TrimSpace(stdout.String()), nil
+}
+
 func (d *DockerDriver) Login(repo, user, pass string) error {
 	d.l.Lock()
 

builder/docker/step_set_defaults.go

@@ -0,0 +1,40 @@
+package docker
+
+import (
+	"context"
+	"strings"
+
+	"github.com/hashicorp/packer-plugin-sdk/multistep"
+)
+
+type StepSetDefaults struct{}
+
+func (s *StepSetDefaults) Run(ctx context.Context, state multistep.StateBag) multistep.StepAction {
+	driver := state.Get("driver").(*DockerDriver)
+	config := state.Get("config").(*Config)
+
+	// Fetch default CMD and ENTRYPOINT
+	defaultCmd, _ := driver.Cmd(config.Image)
+	defaultEntrypoint, _ := driver.Entrypoint(config.Image)
+
+	// Set defaults if not provided by the user
+	hasCmd, hasEntrypoint := false, false
+	for _, change := range config.Changes {
+		if strings.HasPrefix(change, "CMD") {
+			hasCmd = true
+		} else if strings.HasPrefix(change, "ENTRYPOINT") {
+			hasEntrypoint = true
+		}
+	}
+
+	if !hasCmd && defaultCmd != "" {
+		config.Changes = append(config.Changes, "CMD "+defaultCmd)
+	}
+	if !hasEntrypoint && defaultEntrypoint != "" {
+		config.Changes = append(config.Changes, "ENTRYPOINT "+defaultEntrypoint)
+	}
+
+	return multistep.ActionContinue
+}
+
+func (s *StepSetDefaults) Cleanup(state multistep.StateBag) {}