Use ONBUILD Dockerfile instruction

[blueyed]

We should maybe use ONBUILD to make derived Dockerfiles easier to write, where ARG or ENV could be used then to get the Vim versions to be built from. https://docs.docker.com/engine/reference/builder/#/onbuild

[tweekmonster]

That was something I wished existed when I started this. I guess I didn't know it existed 😯