1. Have a private docker registry hosted at registry.gitlab.com or a local install of gitlab with a private registry
2. Create a docker proxy repository in NXRM to the remote gitlab registry URL and an index type that is NOT Docker Hub ( ie. is not HUB)
3. Nexus will fail to authenticate requests sent to to the remote registry. The remote responds with 403 Forbidden
There is a bug where NXRM will only use preemptive auth to the remote if the index type is HUB. For a private remote registry this may not apply.
A change has been made as part of
NEXUS-16718 that causes Nexus to send pre-emptive auth for all docker proxy repository index types instead of just hub. Upgrade to 3.12.0 when it is released.