E2E test for Dependency Proxy for Container Registry

The idea is to have end to end tests for the dependency proxy in different scenarios:

• Pulling an image not present in the dependency proxy
• Pulling an image present in the dependency proxy
• Pulling an image present but stale in the dependency proxy

Context: gitlab-org/gitlab#301049 (closed)

Documentation: https://docs.gitlab.com/ee/user/packages/dependency_proxy/