Github Actions workflows

See the Github documentation for more information on Github Actions in general.

Notes

• https://opensource.google/documentation/reference/github/services#actions mandates using a specific commit for non-Google actions. We use Ratchet to pin specific versions. If you'd like to update an action, you can write something like uses: 'actions/checkout@v4', and then run ./ratchet pin workflow.yml to convert to a commit hash. See the Ratchet README for installation and more detailed instructions.