As pointed by @vlorentz yersteday in a f2f discussion, building the whole
set of docker images associated to each swh service takes too much time.
Moreover there is a lot of duplicated instructions in the service dockerfiles.
This calls for factorization in order to ease future maintenance of that
environment.
So, in order to speed up the docker-compose build process but also to remove
duplicated instructions in dockerfiles, prefer to use a single docker image
containing the full swh stack. Each swh service will run upon it while their
specific configuration (for instance entrypoint) will be picked from the
docker-compose.yml file.