this will avoid a build each time the container starts fresh (notably with tests), as this
is time and resource consuming (notably with node modules which would need to be
downloaded each time).
This patch introduce new docker images, replacing the unmaintained ones which were in
sat_docs.
Images + docker-compose files are provided to use SàT backend and Libervia, but also to
integrate and configure Prosody and SàT PubSub to run end-to-end tests.