Prevent start_for_complement.sh from setting START_POSTGRES to false when it's already set (#16985)

I have a use case where I'd like the Synapse image to start up a
postgres instance that I can use, but don't want to force Synapse to use
postgres as well.

This commit prevents postgres from being started when it has already
been explicitly enabled elsewhere.

docker/complement/conf/postgres.supervisord.conf

@@ -1,7 +1,7 @@
 [program:postgres]
 command=/usr/local/bin/prefix-log gosu postgres postgres
 
-# Only start if START_POSTGRES=1
+# Only start if START_POSTGRES=true
 autostart=%(ENV_START_POSTGRES)s
 
 # Lower priority number = starts first

docker/complement/conf/start_for_complement.sh

@@ -32,8 +32,9 @@ case "$SYNAPSE_COMPLEMENT_DATABASE" in
     ;;
 
   sqlite|"")
-    # Configure supervisord not to start Postgres, as we don't need it
-    export START_POSTGRES=false
+    # Set START_POSTGRES to false unless it has already been set
+    # (i.e. by another container image inheriting our own).
+    export START_POSTGRES=${START_POSTGRES:-false}
     ;;
 
   *)