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/cmd/daemon.go

@@ -63,9 +63,28 @@ func runDaemon(ctx context.Context, configFile *string) func(cmd *cobra.Command,
 		}
 
 		if ls.RequireDocker() {
-			if err := envcheck.CheckIfDockerRunning(ctx, cfg); err != nil {
+			dockerSocketPath, err := getDockerSocketPath(cfg.Container.DockerHost)
+			if err != nil {
 				return err
 			}
+			if err := envcheck.CheckIfDockerRunning(ctx, dockerSocketPath); err != nil {
+				return err
+			}
+			// if dockerSocketPath passes the check, override DOCKER_HOST with dockerSocketPath
+			os.Setenv("DOCKER_HOST", dockerSocketPath)
+			// empty cfg.Container.DockerHost means act_runner need to find an available docker host automatically
+			// and assign the path to cfg.Container.DockerHost
+			if cfg.Container.DockerHost == "" {
+				cfg.Container.DockerHost = dockerSocketPath
+			}
+			// check the scheme, if the scheme is not npipe or unix
+			// set cfg.Container.DockerHost to "-" because it can't be mounted to the job conatiner
+			if protoIndex := strings.Index(cfg.Container.DockerHost, "://"); protoIndex != -1 {
+				scheme := cfg.Container.DockerHost[:protoIndex]
+				if !strings.EqualFold(scheme, "npipe") && !strings.EqualFold(scheme, "unix") {
+					cfg.Container.DockerHost = "-"
+				}
+			}
 		}
 
 		cli := client.New(
@@ -140,3 +159,17 @@ func initLogging(cfg *config.Config) {
 		}
 	}
 }
+
+func getDockerSocketPath(configDockerHost string) (string, error) {
+	// a `-` means don't mount the docker socket to job containers
+	if configDockerHost != "" && configDockerHost != "-" {
+		return configDockerHost, nil
+	}
+
+	socket, found := os.LookupEnv("DOCKER_HOST")
+	if found {
+		return socket, nil
+	}
+
+	return "", fmt.Errorf("daemon Docker Engine socket not found and docker_host config was invalid")
+}