Add support for GitLab Workspaces

Add support for GitLab Workspace.

We had this, and GitPod, set up in the original gitlab-docs project:

• https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/dockerfiles/gitlab-docs-workspace.Dockerfile?ref_type=heads
• https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/.gitpod.yml