diff --git a/Dockerfile b/Dockerfile
index 2b7a638..5d916e3 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,8 +1,13 @@
 FROM resin/raspberrypi-buildpack-deps
 
-# Enable systemd
+# Enable systemd, as Resin requires this
 ENV INITSYSTEM on
 
+# Version number of gateway software
+# (Incrementing this simply forces Docker to flush its cache
+#  and thus forces a full rebuild.)
+ENV TTN_GATEWAY_SOFTWARE 45
+
 # Build the gateway
 COPY build.sh /tmp/build.sh
 WORKDIR /tmp