android/docker: update makefile and dockerfile for build automation

[barnstar]

Updates tailsale/corp#19670

Updated the dockerfile so that it runs the full release build instead of dropping into a shell (where all we were typically doing is running a full release build manually).

The build will now look for JKS_PASSWORD in the environment for completing the signing step without user interaction.

Several smaller recipes added to the makefile for building the docker builder image, running and cleaning it up.