fix: skip service containers with an empty image (#1074)

GitHub Actions skips services whose image evaluates to an empty string, enabling conditional services via expressions like `image: ${{ matrix.image || '' }}`. Here such a service failed the job at `docker create`. Skip them before container creation, using GitHub's log message verbatim, and add a regression test.

Reviewed-on: https://gitea.com/gitea/runner/pulls/1074
Reviewed-by: Nicolas <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>

act/runner/run_context.go

@@ -379,6 +379,13 @@ func (rc *RunContext) startJobContainer() common.Executor {
 
 		// add service containers
 		for serviceID, spec := range rc.Run.Job().Services {
+			// GitHub compatibility: skip services whose image evaluates to an
+			// empty string, enabling conditional services via expressions
+			serviceImage := rc.ExprEval.Interpolate(ctx, spec.Image)
+			if serviceImage == "" {
+				logger.Infof("The service '%s' will not be started because the container definition has an empty image.", serviceID)
+				continue
+			}
 			// interpolate env
 			interpolatedEnvs := make(map[string]string, len(spec.Env))
 			for k, v := range spec.Env {
@@ -417,7 +424,7 @@ func (rc *RunContext) startJobContainer() common.Executor {
 			c := container.NewContainer(&container.NewContainerInput{
 				Name:           serviceContainerName,
 				WorkingDir:     ext.ToContainerPath(rc.Config.Workdir),
-				Image:          rc.ExprEval.Interpolate(ctx, spec.Image),
+				Image:          serviceImage,
 				Username:       username,
 				Password:       password,
 				Cmd:            interpolatedCmd,