[RFC] Automated build of docker image upon successful CI

[tallamjr]

Proposed here is to have travis CI automatically build and push new docker images upon successful completion of CI checks.

Perhaps if the tests are run on the master branch, then it could build and tag with latest, for example:

$ docker build --tag=bluebird .
$ docker tag bluebird turinginst/bluebird:latest
$ docker push turinginst/bluebird:latest

Whereas if the checks are run on a release/* branch, a build a tag with the corresponding release version i.e.

$ docker build --tag=bluebird .
$ docker tag bluebird turinginst/bluebird:1.3.0
$ docker push turinginst/bluebird:1.3.0

What are people's thought's on this set up?

This is linked to https://github.com/alan-turing-institute/nats/issues/5#issuecomment-458490775