Use 'synapse' as a default postgres user in Docker examples

contrib/docker/conf/homeserver.yaml

@@ -50,9 +50,9 @@ listeners:
 database:
   name: "psycopg2"
   args:
-    user: "{{ POSTGRES_USER or "matrix" }}"
+    user: "{{ POSTGRES_USER or "synapse" }}"
     password: "{{ POSTGRES_PASSWORD }}"
-    database: "{{ POSTGRES_DB or "matrix" }}"
+    database: "{{ POSTGRES_DB or "synapse" }}"
     host: "{{ POSTGRES_HOST or "db" }}"
     port: "{{ POSTGRES_PORT or "5432" }}"
     cp_min: 5

contrib/docker/docker-compose.yml

@@ -38,7 +38,7 @@ services:
     image: postgres:10-alpine
     # Change that password, of course!
     environment:
-      - POSTGRES_USER=matrix
+      - POSTGRES_USER=synapse
       - POSTGRES_PASSWORD=changeme
     volumes:
       # You may store the database tables in a local folder..