Clarify labels (#69)

The label will follow the format `label[:schema[:args]]`, and the schema will be `host` if it's omitted. So

- `ubuntu:docker://node:18`: Run jobs with label `ubuntu` via docker with image `node:18`
- `ubuntu:host`: Run jobs with label `ubuntu` on the host directly.
- `ubuntu`: Same as `ubuntu:host`.
- `ubuntu:vm:ubuntu-latest`: (Just a example, not Implemented) Run jobs with label `ubuntu` via virtual machine with iso `ubuntu-latest`.

Reviewed-on: https://gitea.com/gitea/act_runner/pulls/69
Reviewed-by: Zettat123 <zettat123@noreply.gitea.io>
Reviewed-by: wxiaoguang <wxiaoguang@noreply.gitea.io>

runtime/label.go

@@ -0,0 +1,26 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package runtime
+
+import (
+	"fmt"
+	"strings"
+)
+
+func ParseLabel(str string) (label, schema, arg string, err error) {
+	splits := strings.SplitN(str, ":", 3)
+	label = splits[0]
+	schema = "host"
+	arg = ""
+	if len(splits) >= 2 {
+		schema = splits[1]
+	}
+	if len(splits) >= 3 {
+		arg = splits[2]
+	}
+	if schema != "host" && schema != "docker" {
+		return "", "", "", fmt.Errorf("unsupported schema: %s", schema)
+	}
+	return
+}