Travis should build and deploy docker images

[julian-zucker]

Is your feature request related to a problem? Please describe.
Right now, we have to manually build and deploy docker images, so they will drift out of date pretty fast.

Describe the solution you'd like
We should have travis build a docker image (tagged with latest) on a merge into master, and tag docker images for our releases.

Describe alternatives you've considered
I don't know if there are other ways to build docker images automatically, so I haven't really considered alternatives. Centralizing into Travis feels intuitive to me though.