add SERVE_SERVER_WELLKNOWN variable

This config allows matrix-synapse to create a endpoint on
$DOMAIN/.well-known/matrix/server used for federation.
It's a straight forward way (no port opening required)
to enable federation if you are okay having a subdomain
(matrix.example.org) as your homeserver address.

compose.yml

@@ -25,6 +25,7 @@ services:
       - PASSWORD_LOGIN_ENABLED
       - REDACTION_RETENTION_PERIOD
       - ROOT_LOG_LEVEL
+      - SERVE_SERVER_WELLKNOWN
       - SQL_LOG_LEVEL
       - STACK_NAME
       - SYNAPSE_ADMIN_EMAIL