A docker proxy repository returns a 502 response when its remote returns a 401. Normally for proxy repositories this would be working by design, a 401 response indicates a problem with the proxy repository's configuration. However, docker registries rather unfortunately return 401 when they should be returning 404. So this means a proxy repository will be returning 502 constantly, and a remote Nexus Repo instance that proxy's the proxy will auto-block.
Expected: Sadly, docker registries break HTTP protocol here, but we can't fix them, so we will need to fix Nexus Repo. Docker proxy repositories should not auto-block if the remote returns a 401, and we should return a 404 response from the proxy repository.