Fix the Dockerfile: install pip from apt

[magopian]

Fixes #521

[magopian]

Disclaimer: at least locally it seems this fixes the issue, and using this Dockerfile I can launch the kinto server from inside the docker container. However, maybe because of some bad docker configuration, the port forwarding seems to timeout.

If somebody could validate that this fixes the issue for them, that would be awesome ;)

ping @almet @Natim

[leplatrem]

From your branch:

$ sudo docker build .
...
Step 6 : CMD kinto --ini $KINTO_INI migrate && kinto --ini $KINTO_INI start
 ---> Running in 6c86618684bf
 ---> 766361468fef
Removing intermediate container 6c86618684bf
Successfully built 766361468fef

$ sudo docker run 766361468fef
2016-03-30 15:44:52,416 INFO  [venusian][MainThread] kinto 2.1.0.dev0 starting. 

r+

[magopian]

excellent, thanks! I'm landing this