Use dependency proxy

Ref: https://gitlab.com/gitlab-org/gitlab/-/issues/521646+

Why is this change being made?

Starting April 1st, Docker Hub will be imposing rate limits on pulls. The dependency proxy is set up. ✅

To be safe, let's use the Dependency Proxy to ensure no disruptions.

Proof of work

https://gitlab.com/gitlab-org/database-team/gitlab-com-database-testing/-/jobs/9585754536

It is pulling from the dependency proxy, rather than Docker Hub directly.

Using Docker executor with image gitlab.com:443/gitlab-org/dependency_proxy/containers/ruby:3.2.5 ...
Authenticating with credentials from job payload (GitLab Registry)
Pulling docker image gitlab.com:443/gitlab-org/dependency_proxy/containers/ruby:3.2.5 ...
Using docker image sha256:81ae3a563b2e71f072a808fe654e87ad8cdf63eca666a316b0334e6604877412 for gitlab.com:443/gitlab-org/dependency_proxy/containers/ruby:3.2.5 with digest gitlab.com:443/gitlab-org/dependency_proxy/containers/ruby@sha256:681581ccc6d980e05ec4643a9ab209a5e31394950d63aa52a2d12a0c9e28fa1c ...