distribution: official Docker image for CI

[DataDave-Dev]

Context

Some CI environments would rather docker run than install anything on the runner. A tiny image makes becwright check --diff a one-liner in any CI, not just GitHub Actions (GitLab CI, Jenkins, etc.).

Task

• Dockerfile: FROM python:3.12-slim, pip install becwright, git installed (the engine shells out to git), entrypoint becwright
• Publish ghcr.io/datadave-dev/becwright:<version> + :latest from the release workflow
• Document the GitLab CI / generic recipe in documentation/recipes.md:
  docker run --rm -v $PWD:/repo -w /repo ghcr.io/datadave-dev/becwright check --diff origin/main