Docker.DotNet → Testcontainers migration

[louis-z]

• Replace the Docker.DotNet package (unmaintained) with Docker.DotNet.Enhanced (its actively maintained fork, now owned by the Testcontainers org).
• Add the Testcontainers package.
• GitLabDockerContainer.SpawnLocalDockerContainerAsync (used for local test runs) now creates/starts the GitLab container via Testcontainers' ContainerBuilder, instead of ~90 lines of manual container lookup/create/start/health-polling code.
• Credential generation (RunGitLabRailsRunnerAsync) now runs the gitlab-rails runner command differently depending on how the container came to exist:
  • Local runs: execs directly into the container object Testcontainers already holds a reference to.
  • CI runs: GitLab is started by GitHub Actions as a service container, not by our code, so we still use a raw Docker Engine API client (via Docker.DotNet.Enhanced) to find that container and exec into it.

Rationale

• Docker.DotNet has had no releases or fixes in a long time; staying on it was a growing maintenance risk.
• Testcontainers is the purpose-built library for "spin up a container for tests, wait until it's ready, tear it down"; it replaces our hand-rolled orchestration/health-check code instead of just swapping the underlying HTTP client.
• Testcontainers can't attach to a container it didn't create itself, so the CI path (which execs into a pre-existing GitHub Actions service container) still needs a low-level Docker API client. Docker.DotNet.Enhanced fills that one remaining need, and since Testcontainers depends on it internally too, there's no duplicate/conflicting dependency.

Trade-off to know about

• The old code auto-detected when the local GitLab image version changed and removed the stale container before creating a new one.
• The new code uses Testcontainers' WithReuse(true) to keep the container running across local test runs (GitLab takes minutes to boot), but reuse matching is based on name + config.
• If you bump LocalGitLabDockerVersion, you now need to manually remove the old container first:

  docker rm -f NGitLabClientTests