build a docker image for the CI
build a docker image for the CI

Since we have to migrate to CircleCI v2, we are now allowed to
use custom docker image.
That's great as this will allow us to use a more recent version of
Ubuntu (for example, the last LTS) and thus we won't have to use several
PPAs to get up-to-date compilers.

vrancurel/ntl,vrancurel/ntl,vrancurel/ntl