Add dockerfile for Workspace

What does this MR do and why?

Adds a Dockerfile for Workspace usage.

Closes #1689 (closed)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.
2. Build the image: docker build --build-arg ASDF_VERSION="0.12.0" --tag docs:workspace -f dockerfiles/gitlab-docs-workspace.Dockerfile .

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.