Switch main Dockerfile to use binary Debian packages

[wohali]

Overview

Simplify our build environment by simply relying on the pre-built Debian .deb packages for CouchDB, rather than installing all of the build dependencies, building CouchDB from scratch, then removing them all.

Don't Repeat Yourself.

I've also added a VOLUME definition for /opt/couchdb/etc/local.d since we recommend externally mounting this anyway. @kocolosk does this cause any concern for the helm/k8s stuff at all?

Testing recommendations

Travis CI should build the image from scratch and test it on our behalf - if it's green, we're good.

GitHub issue number

Closes #117

Checklist

• Code is written and works correctly;
• Changes are covered by tests;
• Documentation reflects the changes;

[wohali]

Ironically, changes in our Python requirements (python3) means building the dev* images is broken. I'll open a new PR for that, then rebase this once it merges.

[wohali]

Koco has already +1'ed this idea in #117 , so I'm merging.

[wohali]

It would help if I could press the right button.