WIP Refactored/reworked mainly upgrading and slimming Image and added examples

[justb4]

Still work in progress, but reworked the Dockerfile to more current library versions and to slim it from 1GB to about 290MB.
See comments in Dockerfile, basically, using https://github.com/geopython/pygeoapi/blob/master/Dockerfile as example:

• using Debian (Buster) Slim image (Bullseye tried but Python 3.8 incompat with MapProxy).
• avoided building wheels by installing python- packages
• removing build dependency packages.
• compacting ENV and RUN statements

Also added two examples and some optimizations, text edits etc.

The standard example works, so almost good to go.