Add GitLab Pages container

What does this MR do?

Build a Docker image running GitLab Pages.

MR to release-tools to manage the version - gitlab-org/release-tools!1296 (merged)

Related issues

Closes: gitlab-org/charts/gitlab#2275 (closed)

Test infra

I am reusing the same one from gitlab-org/charts/gitlab!1652 (merged), but instead of the omnibus-gitlab VM for Pages, I am spinning up a docker container using this image.

1. Using docker-compose

   version: "3"
   services:
     pages:
       image: registry.gitlab.com/gitlab-org/build/cng/gitlab-pages:gitlab-pages-container
       ports:
         - 80:8090
       volumes:
         - ~/pages/config:/srv/gitlab/config
         - ~/pages/data:/srv/gitlab/pages  # Kinda unnecessary, but Pages has a strong requirement on this being available to be `chdir`d into.

2. Using configuration file ~/pages/config/pages-config

   listen-proxy=0.0.0.0:8090
   pages-domain=<pages domain>
   pages-root=/srv/gitlab/pages
   log-verbose
   redirect-http=false
   use-http2=true
   artifacts-server=<GitLab API v4 URL>
   artifacts-server-timeout=10
   gitlab-server=<GitLab URL>
   api-secret-key=/srv/gitlab/config/.gitlab_pages_secret
   domain-config-source=gitlab

3. Using API secret ~/pages/config/.gitlab_pages_secret with content dGhpc2lzYWJvZ3VzdGhpcnR5dHdvY2hhcmFjdGVycHc= (base64 encoded version of thisisabogusthirtytwocharacterpw)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Integration tests added to GitLab QA
• The impact any change in container size has should be evaluated