services: add Synapse

Synapse is the Matrix server software.

This needs some manual setup, launching `docker-compose exec synapse sh`
followed by `register_new_matrix_user -c /data/homeserver.yaml
https://matrix.belanyi.fr/` inside the container.

docker-compose.yml

@@ -326,3 +326,14 @@ services:
     expose:
       - 5000
     user: 1000:1000
+
+  synapse:
+    image: matrixdotorg/synapse
+    restart: unless-stopped
+    container_name: synapse
+    env_file:
+      - ./matrix/matrix.env
+    volumes:
+      - ./matrix:/data
+    ports:
+      - 8008:8008