Migrate Dockerfile to Debian

Thom Wiggers
2017-03-29T20:17:23+02:00

Rewrote the Dockerfile to be Debian based

**Extensive testing needed**
* [x] @lscholten
* [x] @twiggers
* [x] Normal stuff works
* [x] Uploading images works (Pillow)
* [x] unit tests work (`docker-compose run web test`) (cover...