Here’s the workflow

name: "Test in a Raku container"
on: [ push, pull_request ]
jobs:
  test:
    runs-on: ubuntu-latest
    container:
      image: ghcr.io/jj/gha-raku:latest
      credentials:
        username: ${{ github.actor }}
        password: ${{ secrets.GITHUB_TOKEN }}
    steps:
      - name: Checkout
        uses: actions/checkout@v2
      - name: Install modules
        run: |
          raku -e 'say $*REPO.repo-chain'
          HOME=/home/raku && zef install --verbose --deps-only .
  - name: Test
    run: zef --debug test .

It’s run here
with this error

/usr/bin/docker --config /home/runner/work/_temp/.docker_3a31fecb-bf57-483d-93b9-47e99b2057b0 login ghcr.io -u JJ --password-stdin
  /usr/bin/docker --config /home/runner/work/_temp/.docker_3a31fecb-bf57-483d-93b9-47e99b2057b0 pull ghcr.io/jj/gha-raku:latest
  Error response from daemon: manifest unknown
  Warning: Docker pull failed with exit code 1, back off 6.182 seconds before retry.

The image is public and, besides, this repo is granted permission, so I don’t even get why do I need to log in there…