Support specifying command for services (#50)

This PR is to support overwriting the default `CMD` command of `services` containers.

This is a Gitea specific feature and GitHub Actions doesn't support this syntax.

Reviewed-on: https://gitea.com/gitea/act/pulls/50
Reviewed-by: Jason Song <i@wolfogre.com>
Co-authored-by: Zettat123 <zettat123@gmail.com>
Co-committed-by: Zettat123 <zettat123@gmail.com>

pkg/model/workflow.go

@@ -522,6 +522,9 @@ type ContainerSpec struct {
 	Args        string
 	Name        string
 	Reuse       bool
+
+	// Gitea specific
+	Cmd []string `yaml:"cmd"`
 }
 
 // Step is the structure of one step in a job