Support workflows with private images

[kevbook]

Example workflow

jobs:
  check:
    runs-on: ubuntu
    container:
      image: ghcr.io/private-org/ci-image:latest
      credentials:
        username: ${{ github.actor }}
        password: ${{ secrets.GITHUB_TOKEN }}

For a container job, agent-ci does: docker.pull(image, callback) // no authconfig, no cache guard

• It never reads the credentials block — pulls the image anonymously
• There is no private-registry auth anywhere
• It pulls every run even though the image is already cached locally

Let me know if you need help w/ PR

[peterp]

Oooh, will fix. thansk fro the report!