Rewrite partly the Dockerfile so that:

- The image is derived from continuumio/miniconda3 rather than Ubuntu (maybe it is not the right call - no clue one way or another, but we probably have shave off a bit of space. The miniconda one is apparently base on Debian).
- The project is no longer cloned from gitlab, but is rather read from the host (the command line is therefore a bit longer and is documented in the README). The reason for doing so is that the image is lighter and fork may build their own image from their actual master branch rather than silently taking the one from the main project.

I have also removed several lines that seems to add nothing; maybe I was wrong on some of these calls.

The terminal where the `run` command is run is no longer silent; I don't know what triggered the difference here.
1 job for master in 7 minutes and 56 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #1122628
build_docker

00:07:56