Added a README.md for curious students

nexus3/README.md

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+This Dockerfile is a combination of the two Dockerfiles (locked at the commit):
+
+* https://github.com/sonatype/docker-nexus3/blob/37a9fcd2b9b7f13d3ca486981732168f98b12199/Dockerfile
+* https://github.com/sonatype/docker-nexus3/blob/37a9fcd2b9b7f13d3ca486981732168f98b12199/Dockerfile.rh.el
+
+Both Dockerfiles use Chef recipes to install Nexus.  These Chef recipes are
+downloaded during the build by the Dockerfile.
+
+The first Dockerfile uses the ubi8/ubi image, but is missing LABEL(s) that OpenShift
+and Kubernetes use.  This Dockerfile also uses an ENTRYPOINT to account for OpenShift
+assigning a random UID for each container.
+
+The second Dockerfile includes the LABELs needed by Kubernetes and OpenShift,
+but does not use UBI and does not indicate how it handles the random UID.
+Testing an image off of this Dockerfile shows that there is no issue with the
+image running.
+