Tweak build script and docker setup

Tweak build script and docker setup

- Output deb files to debs/ folder by default.
- Use $HOME/.termux-build for build folder by default (changed
  from previous $HOME/termux).
- Make scripts/run-docker.sh re-use a single container instead of
  creating new ones for each run.
- Avoid mounting the whole Termux build folder in docker. This
  fixes #294, mmap not supported host folders under OS X.
- Add a scripts/update-docker.sh utility for checking for an
  updated docker image..
- Make build-package.sh support the [-a ARCH] flag to specify arch.
- Make aarch64 the default arch (from previous arm).
- Try to improve the README a bit.