feat(docker): add setup-ccbr-actions composite action and Docker build workflow

[Copilot]

Composite actions in this repo always pip-install ccbr_actions from GitHub. This PR adds a Docker image for ccbr_actions and a composite action that uses it when available, falling back to pip only when no matching image exists.

New: Dockerfile + build workflow

• Dockerfile: python:3.11-slim base; installs ccbr_actions via pip at CCBR_ACTIONS_VERSION build-arg (defaults to main). Includes print_versions.sh and prepare_docker_build_variables.sh scripts.
• .github/workflows/build-docker-ccbr-actions.yml: builds and pushes nciccbr/ccbr_actions:{tag} to DockerHub on release: published; manually triggerable with optional push flag.

New: setup-ccbr-actions composite action

Replaces the actions/setup-python + pip-install pattern used across composite actions. Logic:

1. Queries DockerHub API for nciccbr/ccbr_actions:{ccbr-actions-version}
2. HTTP 200 → docker path: docker pull, outputs install-method=docker + docker-image=nciccbr/ccbr_actions:{version}
3. Otherwise → pip path: actions/setup-python + pip install, outputs install-method=pip

Outputs let callers branch on which method is active.

Updated: build-docker action

• Replaces inline setup-python + pip-install with ./setup-ccbr-actions
• Converts shell: python steps (prepare_vars, check_tag_staleness) to shell: bash with conditional dispatch:
  • docker: docker run with $GITHUB_ENV / $GITHUB_OUTPUT bind-mounted so Python can write outputs
  • pip: run Python directly on the host
• run_script_in_container step extracts print_versions.sh and the default config file from the ccbr_actions image via docker run --entrypoint sh when in docker mode

- name: Set up ccbr_actions
  id: setup-ccbr-actions
  uses: CCBR/actions/setup-ccbr-actions@main
  with:
    ccbr-actions-version: v0.8.0   # has a DockerHub image → docker path
    # ccbr-actions-version: my-branch  # no image → pip fallback