Docker: update cross-compiler Dockerfile and build script

Updated so that it can build Haiku R1 Beta 4. The intention is to use this to
create repeatable builds for Rust builds. The script has been tweaked to use
the latest stable debian as base image, to include the (now-required)
libzstd-devel package, and the g++-multilib package to support building 32 bit
in a 64 bit host. It also moves to python 3.x.

Additionally, the image now supports specifying custom git tags/branches for
the buildtools and Haiku itself, so that specific hrevs or branches can be
built

Change-Id: Ia43089ff7271256fdb6603bd4400ee3b9228ea1b
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5965
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>

docker: tools for creating a container with Haiku cross-compilers.

* This also includes the package tool for easy use of installing
dependencies in a container.
* Pre-built docker images are also available at
https://hub.docker.com/r/haiku/cross-compiler/