feat: split image flag into image name and tag (#17)

README.md

@@ -86,14 +86,25 @@ jobs:
       - uses: Example/example-action@a35f...b316
 ```
 
-If you want your action to use a different image, then you can specify one manually in `action.yml`.
+If you want your action to use a specific image tag, then you can set one manually in `action.yml`.
 
 ```diff
 runs:
   using: docker
   image: docker://ghcr.io/joshdk/actions-docker-shim:v0.0.2
 +  args:
-+    - --image=ghcr.io/example/example-action:snapshot
++    - --image-tag=snapshot
+```
+
+If your image isn't named the same as your action repository, that can be overridden as well.
+
+```diff
+runs:
+  using: docker
+  image: docker://ghcr.io/joshdk/actions-docker-shim:v0.0.2
+  args:
+    - --image-tag=snapshot
++    - --image=example/some-other-image
 ```
 
 ### Authentication

main.go

@@ -24,17 +24,21 @@ func main() {
 
 //nolint:forbidigo,wsl
 func mainCmd() error {
-	var image string
-	flag.StringVar(&image, "image", "", "ghcr.io image to run")
+	var imageRepo string
+	flag.StringVar(&imageRepo, "image", "", "ghcr.io image to run")
+	var imageTag string
+	flag.StringVar(&imageTag, "image-tag", "", "ghcr.io image tag to run")
 	var tokenEnv string
 	flag.StringVar(&tokenEnv, "token-env", "", "env var to use for github token")
 	flag.Parse()
 
-	if image == "" {
-		repository := os.Getenv("GITHUB_ACTION_REPOSITORY")
-		ref := os.Getenv("GITHUB_ACTION_REF")
-		image = fmt.Sprintf("ghcr.io/%s:%s", strings.ToLower(repository), ref)
+	if imageRepo == "" {
+		imageRepo = os.Getenv("GITHUB_ACTION_REPOSITORY")
 	}
+	if imageTag == "" {
+		imageTag = os.Getenv("GITHUB_ACTION_REF")
+	}
+	image := fmt.Sprintf("ghcr.io/%s:%s", strings.ToLower(imageRepo), imageTag)
 
 	var token string
 	if tokenEnv != "" {