Change Plausible to not bind to port 80 by default

This is done in preparation for adding reverse-proxy configurations
Unfortunately a later docker-compose file cannot *remove* a port
binding, so it has to be removed in the root file.
If there is interest, the 80:8000 binding can be re-added in a
new docker-compose overwrite file.

docker-compose.yml

@@ -28,7 +28,7 @@ services:
       - plausible_events_db
       - mail
     ports:
-      - 80:8000
+      - 8000:8000
     env_file:
       - plausible-conf.env