Setup automatic docker build on new release

[marcj]

Everytime we tag a new release, we should build the docker image automatically. Dunno how at the moment. Maybe we can use Github's API and detect when a new tag has been added and then kickstart some build on a buildsystem.

[k911]

You can use any CI for that, with deploy script configured to build docker and push to registry whenever build is triggered on tag.

Example .travis.yml deploy script definition:
https://github.com/knit-pk/api-v1-php/blob/f2fa73946ade1d76e1ac4fa8204055fe3d1dceab/.travis.yml#L107

And deploy script itself: https://github.com/knit-pk/api-v1-php/blob/develop/deploy/docker-app-push.sh