provide only one defacto official Nexus 3 docker image

Background
https://hub.docker.com/r/sonatype/docker-nexus3/ is now the automated official build?

https://hub.docker.com/r/sonatype/nexus3/ is some manually created thing.

The descriptions for each of these projects do not make it clear which should be avoided and which should be relied upon. This causes customer confusion and support headaches.

Please clearly change the descriptions of each of these projects and/or remove the https://hub.docker.com/r/sonatype/nexus3/ one if that is going to remain a deprecated one.

Leaving the old nexus3 one around without very clear instructions and some clear deprecation is NOT a good option.

Acceptance

• Gut the nexus3 project and leave it with a README that points to docker-nexus3.