Add a site building Docker image

What does this MR do and why?

To solve #182 (closed), create a Docker image purely for site builds.

How to set up and validate locally

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/technical-writing/docs-gitlab-com/-/blob/main/doc/setup.md.
2. Check out this branch.
3. To build the Docker image, run docker buildx build --file dockerfiles/docs-gitlab-com-builder.Dockerfile --build-arg="HUGO_VERSION=0.142.0" --platform=linux/amd64 ..
4. To run the Docker image, run docker run -ti --rm --platform=linux/amd64 <Docker image SHA> bash.
5. Within the running Docker container, run these commands:
   • git clone https://gitlab.com/gitlab-org/technical-writing/docs-gitlab-com.git
   • cd docs-gitlab-com/
   • make clone-docs-projects
   • make setup
   • yarn build

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.