Images should reflect language versions

[andystanton]

Currently the versions of Docker images reflect changes made to the Dockerfiles. This seems a pointless thing to keep track of when the really important thing is the version of the language the Dockerfile contains.

For example, rather than lang-cpp:1.0.2 it is more useful to have lang-cpp:11 which could be an alias for even more detailed information like lang-cpp:clang-3.4 or lang-cpp:gcc-4.9.3.

This requires some thinking.