Releases and update for newer actions?

[vsoch]

Hey spack-github-action maintainers! I'm wondering if it would make sense to:

• update this for the newer GitHub actions (I think this syntax is deprecated now, but I haven't tried)
• have proper releases so the user doesn't have to target a branch

I have a use case of wanting to quickly install spack, and this would be perfect for it.

[vsoch]

But I'm not sure what the specific use case for the action would be here, because I can easily use a spack container base to have a quick install of spack, e.g.,

jobs:
  spack-containerize:
    runs-on: ubuntu-latest
    container:
      image: spack/ubuntu-bionic
      options: --entrypoint /bin/bash
    steps:
      - name: Checkout
        uses: actions/checkout@v2        
      - name: Generate spack package Dockerfile
        run: |
           spack containerize

What are the intended use cases for the action?