Update README, Dockerfiles to pass build, promote smoother on-boarding

README:

• add detailed instructions on installing and compiling
• provide workarounds for gotchas setting up emulator on 64-bit debian
• provide updated dockerfile & offer instructions for building in docker

Docker:

• move dockerfiles to registry contained w/in bitmask_android repo
• provide separate dockerfiles for sdk (base), ndk, and emulator images
• resolve issues causing containerized build to fail, including:
  • upstream ics-openvpn conflict with clang (:. transitively with ndk > r12b)
  • (subtle!) missing dependency of ndk r12b on file (message: "missing file" meant a missing program called "file" not an actual missing file)
  • missing locales causing errors in copyIcsOpenVPNFiles build step (resolved by generating all locales in dockerfile)
  • silent failure in sdknmanager's isntallation of platforms;android-23 during image build causing install failure in remote (but not local!) containers