[WM] switch to supervisor base image

We're removing supervisor from the "base" image, but this still needs it.
[WM] switch to oracle-jdk-7-supervisor base image

I added some explanatory comments about this so future me won't hate present me.
[WM] switch to a better Docker base image

This allows one Docker inheritance chain which is much saner.
Merge pull request #220 from votinginfoproject/from-supervisor-base-image

[WM] switch to supervisor base image