Dockerfile for build environment

It would be possible to codify the build dependencies and instructions in something a bit lighter weight than a VM. This Dockerfile could be released with the source, and would allow users to easily make small changes to the build, such as patching parts of a tool or library, or changing the version of python that gdb is dynamically linked against (in my case). Depending on your internal tooling, this could simplify the release process as well.

I've prototyped this by just following the instructions in the manual, and have come up with Dockerfiles for trusty and xenial. Both seem to be able to produce valid toolchains.