Use the docker multi-stage build functionality to perform the Retro68 build in a
temporary build image, and then copy the final build directory into a fresh
release image. This reduces the final release image size from 9.2GB to 2.6GB.
Update Dockerfile to use Ubuntu 20.04 and the updated package list from
azure-pipelines.yml. Also explicitly include the g++ package into the container
without which configure will error out with a 'configure: error: C++ preprocessor
"/lib/cpp" fails sanity check' error.
