Improve tagging of base image

To avoid duplicated dependency installs among other Docker images, tag the base image in a way that downstream images must opt-in. This avoids accidentally breaks where base often constantly changing.

This also will make the image easier to find in the registry: https://gitlab.com/gitlab-org/gitlab-docs/container_registry (not just under Root image).

I'm attempting to make this deploy when a change to the base Dockerfile is merged, rather than having to deploy manually. This should save having to remember to run a manual step.

I could see the result of this merged MR being used in the linting images and the single image.

Closes: #1206 (closed).