Support YAML Anchors

Describe the enhancement Support YAML anchors in in workflow files

Code Snippet

  - env: &docker_cred
      DOCKER_USER: ${{ secrets.DOCKER_USER }}
      DOCKER_PASS: ${{ secrets.DOCKER_PASS }}
# ...
  - env:
      <<: *docker_cred

OR

anchors:
 docker_hub_credentials: &docker_hub_credentials
   credentials:
     username: my-dockerhub-username
     password: ${{ secrets.DOCKER_HUB_PASSWORD }}

 postgres_service: &postgres_service
   postgres:
     image: mdillon/postgis:9.6
     <<: *docker_hub_credentials

Additional information This has been asked for before, in other places

• Issue from starter-workflows
• highly-upvoted comment from compose actions issues

Note to imlementors

This is generally handled by YAML parsers automatically. Perhaps it has been disabled, or a parser that does not support it is being used. This might be as simple as switching YAML parsers.

[freitasmurillo]

3+ years of people complaining and u don’t take any action? Either, say u won’t do or do it already. not a big deal to implement.

[kyeotic]

@DanySK (and to a lesser degree @freitasmurillo), while I appreciate the support on issues, since upvotes help show priority, I don’t think hurling abuse at the maintainers or anyone else is going to get this done any faster. If either of you are professional software developers I’m sure you can relate to working on project’s whose feature requests exceed your team’s bandwidth. There are 300 open issues on this repository and some of them describe actual bugs. Surely this is not the highest priority ticket.

Please keep in mind there is a human on the other side reading these messages. Consider how you might react to someone who said as much to you.