Add Brotli to site builder image

What does this MR do and why?

As a follow up to !1173 (merged), we can install Brotli into the site builder container image rather than installing it every time the relevant CI/CD jobs are run. This is a nice housekeeping step.

The next time a new site builder container image is deployed, we can remove the Brotli install step from:

• https://gitlab.com/gitlab-org/technical-writing/docs-gitlab-com/-/blob/563e9f34709ce66665380270e7aeab7bb7fe174d/.gitlab/ci/build.gitlab-ci.yml#L38.
• https://gitlab.com/gitlab-org/technical-writing/docs-gitlab-com/-/blob/563e9f34709ce66665380270e7aeab7bb7fe174d/.gitlab/ci/build.gitlab-ci.yml#L91

How to set up and validate locally

1. Configure a local GitLab Docs environment.
2. Check out this branch.
3. Run docker buildx build --file dockerfiles/docs-gitlab-com-builder.Dockerfile --build-arg HUGO_VERSION_NEXT="0.150.1" --progress="plain" --no-cache . to build the container image.
4. Run docker run -ti --rm <container image SHA> sh.
5. In the running container, run brotli --version.

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.