Provide clearer on-premise build instructions, as pre-built images rarely match available z-streams

[brgavino]

While the on-premise instructions in the README have a blurb about on-premise build, the actual instructions are unclear. Since z-streams are more frequent, installable or upgradable z versions may not match the pre-built versions hosted on the container registry.

• It is assumed that the default route is enabled for the cluster registry -> need to list this as a pre-requisite (or ensure that administrator has setup a registry to be configured)
• The configMap that needs to be created for the Module needs to be defined, to ingest the dockerfile as a value