Docker: do not reference any paths outside /docker/ when building (for the Hub)

Sorry for this new MR; the docker automated build didn't work, and I had to fix a few things for this.

It seems Docker's Hub (called the Hub then) puts the Docker build context within the directory that contains the Dockerfile, so ./docker in our case. Since Docker can't use files outside the build context directory, we couldn't just copy files from ../: I've reverted to cloning the repository from github (and removing .git after cloning), since it is github that triggers the Hub push after a github's push anyways.

Can anybody have a quick look? I've tested it, and automated builds seem to work well after this 👍