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/jobparser/model.go

@@ -168,6 +168,7 @@ type ContainerSpec struct {
 	Volumes     []string          `yaml:"volumes,omitempty"`
 	Options     string            `yaml:"options,omitempty"`
 	Credentials map[string]string `yaml:"credentials,omitempty"`
+	Cmd         []string          `yaml:"cmd,omitempty"`
 }
 
 type Strategy struct {