Support building different dockerfiles for different OSes
Related to #1634
We want to provide an image based on RHEL, so we need to change our build setup a bit to support different Dockerfiles
We have also received requests for ARM based docker images: #1967
And while we aren't committing to any yet, this work would allow us to more easily provide such things in the future.
The work is already WIP here: !1105 (closed)