Document function container requirements including running as non-privileged user

[shaunsmith]

Recent changes require functions to run as an unprivileged user. Fn Server was updated and the base FDK images were updated to align with this requirement. This change has at least two doc impacts:

1. Describe this constraint so that users are aware that it is there
2. Incorporate this constraint into the description/tutorials of how to build a function from a custom container.

Fn Server PR: fnproject/fn#1442
Java FDK PR: fnproject/fdk-java#194
There are other PRs for each FDK.