Document how to build stack in "air gapped" environment

[scottkurz]

(Copied over from appsody/stacks#771)

Some things to focus on:

• Setting up the Maven repository including openliberty runtime/feature installation (including service / ifixes potentially)
• The development image adding maven: kabanero/ubi8-maven:0.3.1 (maybe mention how this is a generic Java/Maven image not especially tied to Open Liberty.

I also wonder if there's any value in defining an "offline Maven" path through the build? By that I mean, instead of assuming there's an internal Maven repository, assume there's no other Maven repository beyond that built into the stack, i.e. all the dependencies will be built into the stack up front.

Worth reading this issue: eclipse-che/che#15518 (comment)