Update the android build docker image:
- rebase on ubuntu 22.04;
- add tooling required to sign APKs;
- changes to make the container re-usable;
- change to a multi-stage build to keep the image size smaller;
- generic improvements to the Dockerfile
Also update the example script for how to use the container.
Signed-off-by: Michael Keller <github@ike.ch>
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