Add support for PUID/PGID environment variables

These arefor setting the user/group to drop privileges to.

Closes #2441. Thanks to matthewparkes.

docker/local/docker-entrypoint.sh

@@ -3,8 +3,14 @@ set -e
 
 # Set permissions
 user="$(id -u)"
+if [ "$PUID" = "" ]; then
+	PUID="mosquitto"
+fi
+if [ "$PGID" = "" ]; then
+	PGID="mosquitto"
+fi
 if [ "$user" = '0' ]; then
-	[ -d "/mosquitto" ] && chown -R mosquitto:mosquitto /mosquitto || true
+	[ -d "/mosquitto" ] && chown -R ${PUID}:${PGID} /mosquitto || true
 fi
 
 exec "$@"