From what I can tell, it's currently not possible to negate the value calculated by is_default_branch. This makes it harder to disable a tag for all branches other than the default branch.

This could be supported by either adding a specific is_not_default_branch or with a not (...) helper (though given that is_default_branch is the only Global expression that this would work with, it seems rather pointless). An alternative, but probably larger change could be to support a disable attribute that reverses the logic used by enable.

If I've overlooked a way to do this that already exists, please let me know! Happy to work on a PR to support this if some guidance on preferred solution can be given.

Example workflow

The example workflow below has two tags - one which uses is_default_branch and adds the this-is-main- prefix, and a second which adds the this-is-not- prefix. Currently the only way to do this is to fall back to standard Github Workflow expressions.

name: test-docker-tags

on:
  push:
    branches:
      - '**'

jobs:
  docker:
    runs-on: ubuntu-latest
    steps:
      -
        name: Docker meta
        id: meta
        uses: docker/metadata-action@v4
        with:
          images: |
            name/app
          tags: |
            type=ref,event=branch,enable={{is_default_branch}},prefix=this-is-main-
            type=ref,event=branch,enable=${{ github.ref != format('refs/heads/{0}', 'master') }},prefix=this-is-not-