CI: use Dependency Proxy when available
This sets up CI to allow the use of the GitLab Dependency Proxy which caches images pulled from DockerHub, in order to bypass rate-limiting.
The DOCKER_REGISTRY_URL variable is set dynamically by the check_dependency_proxy_access job defined in dependency_proxy.yml such that only pipelines triggered by users with the requisite access will be configured to use the proxy, while all others will continue to pull from DockerHub as before.
When DOCKER_REGISTRY_URL is pre-set in a project's CI/CD variable settings, the extra job is skipped and the dependency proxy is used always, unconditionally.
To avoid breaking CI pipelines on 3rd-party GitLab instances, we only include the dependency proxy template on gitlab.tpo