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/pkg/envcheck/docker.go

@@ -8,17 +8,15 @@ import (
 	"fmt"
 
 	"github.com/docker/docker/client"
-
-	"gitea.com/gitea/act_runner/internal/pkg/config"
 )
 
-func CheckIfDockerRunning(ctx context.Context, cfg *config.Config) error {
+func CheckIfDockerRunning(ctx context.Context, configDockerHost string) error {
 	opts := []client.Opt{
 		client.FromEnv,
 	}
 
-	if cfg.Container.DockerHost != "" {
-		opts = append(opts, client.WithHost(cfg.Container.DockerHost))
+	if configDockerHost != "" {
+		opts = append(opts, client.WithHost(configDockerHost))
 	}
 
 	cli, err := client.NewClientWithOpts(opts...)