Add Dockerfile for easy building with custom API key support

[Pablo-Merino]

This PR adds a Dockerfile, along with some build scripts, to easily build a local copy using Docker. It also has the ability to use a custom API key & User Agent (based on this Google Collab script).

This is my first contribution on the project, so I apologize if something's wrong! Thanks a lot for such awesome app!

[linrav]

Thank you for this effort, it really did make this mostly effortless. Apologies for not being familiar with feedback or contribution model, but my two cents after trying this out:

• Critical issues:
  • Trying on my machine the .build.sh script starts an interactive bash shell which immediately exits after build is complete and the line to copy the apk can never succeed
  • dockerfile/build steps are architecture specific, only succeeds on amd64 / fails on apple silicon notably
• Less important:
  • Build can be sped up quite a lot by using a different base or better layer caching

I was able to make minor modifications to correct this, and apologies if this is just my dev environment's nuance, but that in itself makes me inclined to push for more hardening of this process