Update metadata-syntax-for-github-actions.md (#3468)

* Update metadata-syntax-for-github-actions.md

I tried to use another name than Dockerfile, the build fails saying:

`docker: invalid reference format: repository name must be lowercase.`

* Update content/actions/creating-actions/metadata-syntax-for-github-actions.md

Thanks, I struggled to find the correct words and not change the text too much

Co-authored-by: hubwriter <[email protected]>

Co-authored-by: hubwriter <[email protected]>

Author: mambax

content/actions/creating-actions/metadata-syntax-for-github-actions.md

@@ -299,7 +299,7 @@ runs:
 
 #### `runs.image`
 
-**Required** The Docker image to use as the container to run the action. The value can be the Docker base image name, a local `Dockerfile` in your repository, or a public image in Docker Hub or another registry. To reference a `Dockerfile` local to your repository, use a path relative to your action metadata file. The `docker` application will execute this file.
+**Required** The Docker image to use as the container to run the action. The value can be the Docker base image name, a local `Dockerfile` in your repository, or a public image in Docker Hub or another registry. To reference a `Dockerfile` local to your repository, the file must be named `Dockerfile` and you must use a path relative to your action metadata file. The `docker` application will execute this file.
 
 #### `runs.env`