[wip] build docker from local source

[de-code]

@lfoppiano this is a potential revision that we can discuss next week (I thought it's good to have something to look at).

Advantages:

• can be built from local source and no direct link to github repo
• docker can use cache (although in the current version it is not yet very effective unless we can cache the gradle dependencies, e.g. by mounting the .gradle volume)
• as far as I know, that is how docker is usually used (otherwise the Dockerfile could be outside the repo)
• changes to the project structure / build requirements can be reflected in the Dockerfile together with the code
• you could easily integrate it into the CI build workflow to push docker images for commits

Changes to release:

• rather than building the release image locally, it might be better do that on a CI machine
• otherwise you could instead have a very simple script that checks out a particular version and then builds the docker image (instead of doing the git checkout as part of the docker build)

[lfoppiano]

Thanks @de-code. 👍

[coveralls]

Coverage remained the same at 38.684% when pulling 9338013 on de-code:local-dockerfile into a529f40 on kermitt2:master.