docker: certificate generation + various improvments: - certificate is now auto-generated on first prosody launch is there is not already one - certificate generated on build is removed to avoid image-wide certificate - generated certificates are stored in sat_data - data image is now based on prosody which is itslef based on sat_pubsub - prosody configuration is moved to /etc/prosody/prosody_sat_cfg, and stored in sat_data - building order changed to adapt to new images hierarchy - libervia default configuration set to both without redirection (and with a security warning)

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