There's also another workaround to the ones mentioned by other posters: You could install your compilers, do your build, and clean up all your build tools within just one shell script invoked by a RUN line in Dockerfile. It's not pretty but it works.
By the way: there is an open request for contributions to help improve this. The core Docker team very much wants to improve squashing in build, but it's a matter of time and resources.
If somebody cares enough to take the time to carry a design proposal then a patch, we would be happy to support that effort!
