Transition into a Docker container

[AndersSundman]

When building embedded code, bazel toolchain transitions are very useful. We build a PC application with one toolchain and transition to an embedded toolchain to build the firmware that we then embed into the PC application as a firmware upgrade blob before the build is over. Nice!

BUT... what if you can't run all your toolchains on the same machine? Some embedded toolchains only work on Windows, others only work on Linux. It would be awesome if we could start building on Windows and when we need to transition to the Linux embedded toolchain, bazel can not only transition the toolchain but also jump into a Linux docker container to use it.