Upload nightly docker image from travis

We have a daily build on master for the docker image, we can do the same for pre and upload the prerelease to docker hub or maybe trigger a docker build on the docker cloud from there directly.

Opened via [gitter](https://gitter.im/coala/coala/?at=586ea540af6b364a29357417) by @sils

Start by tracing the existing daily Docker image build on master in the Travis configuration and the current Docker Hub publishing path. Decide whether the prerelease should be uploaded to Docker Hub or built through Docker Cloud, then verify that a daily build from pre produces the expected prerelease image.

Written by the indexing model from the issue text.