`SAT_TEST_ENV_E2E` is set to `1` to indicate that e2e environment requirement are
available (notably specific accounts and hosts created). The e2e tests needing it won't be
launched if this variable is not set.
A network alias is set for the backend at `sat.test` (to have a tld available, needed for
some network libraries like OpenSSL).
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.