Improve the usage of docker_host configuration (#260)

Follow #242, #244
Fixes #258

Users could use `docker_host` configuration to specify which docker daemon will be used by act_runner.
- If `docker_host` is **empty**, act_runner will find an available docker host automatically.
- If `docker_host` is **"-"**, act_runner will find an available docker host automatically, but the docker host won't be mounted to the job containers and service containers.
- If `docker_host` is **not empty or "-"**, the specified docker host will be used. An error will be returned if it doesn't work.

Reviewed-on: https://gitea.com/gitea/act_runner/pulls/260
Co-authored-by: Zettat123 <zettat123@gmail.com>
Co-committed-by: Zettat123 <zettat123@gmail.com>

internal/app/run/runner.go

@@ -179,27 +179,28 @@ func (r *Runner) run(ctx context.Context, task *runnerv1.Task, reporter *report.
 		BindWorkdir:    false,
 		ActionCacheDir: filepath.FromSlash(r.cfg.Host.WorkdirParent),
 
-		ReuseContainers:      false,
-		ForcePull:            false,
-		ForceRebuild:         false,
-		LogOutput:            true,
-		JSONLogger:           false,
-		Env:                  r.envs,
-		Secrets:              task.Secrets,
-		GitHubInstance:       strings.TrimSuffix(r.client.Address(), "/"),
-		AutoRemove:           true,
-		NoSkipCheckout:       true,
-		PresetGitHubContext:  preset,
-		EventJSON:            string(eventJSON),
-		ContainerNamePrefix:  fmt.Sprintf("GITEA-ACTIONS-TASK-%d", task.Id),
-		ContainerMaxLifetime: maxLifetime,
-		ContainerNetworkMode: container.NetworkMode(r.cfg.Container.Network),
-		ContainerOptions:     r.cfg.Container.Options,
-		Privileged:           r.cfg.Container.Privileged,
-		DefaultActionsURLs:   parseDefaultActionsURLs(taskContext["gitea_default_actions_url"].GetStringValue()),
-		PlatformPicker:       r.labels.PickPlatform,
-		Vars:                 task.Vars,
-		ValidVolumes:         r.cfg.Container.ValidVolumes,
+		ReuseContainers:       false,
+		ForcePull:             false,
+		ForceRebuild:          false,
+		LogOutput:             true,
+		JSONLogger:            false,
+		Env:                   r.envs,
+		Secrets:               task.Secrets,
+		GitHubInstance:        strings.TrimSuffix(r.client.Address(), "/"),
+		AutoRemove:            true,
+		NoSkipCheckout:        true,
+		PresetGitHubContext:   preset,
+		EventJSON:             string(eventJSON),
+		ContainerNamePrefix:   fmt.Sprintf("GITEA-ACTIONS-TASK-%d", task.Id),
+		ContainerMaxLifetime:  maxLifetime,
+		ContainerNetworkMode:  container.NetworkMode(r.cfg.Container.Network),
+		ContainerOptions:      r.cfg.Container.Options,
+		ContainerDaemonSocket: r.cfg.Container.DockerHost,
+		Privileged:            r.cfg.Container.Privileged,
+		DefaultActionsURLs:    parseDefaultActionsURLs(taskContext["gitea_default_actions_url"].GetStringValue()),
+		PlatformPicker:        r.labels.PickPlatform,
+		Vars:                  task.Vars,
+		ValidVolumes:          r.cfg.Container.ValidVolumes,
 	}
 
 	rr, err := runner.New(runnerConfig)