debian-stretch-titan: Ensure rebuild on changes in repostory

Let's use the well-known hack of adding the "Release" file
of the repository to ensure docker will invalidate the cache if the
repository has changed since the last build.

Change-Id: I54565a4d52221be7e6ded5a339bbf005b58f5998

debian-stretch-titan/Dockerfile

@@ -13,6 +13,7 @@ RUN	apt-key add /tmp/Release.key && \
 	rm /tmp/Release.key && \
 	echo "deb " $OSMOCOM_REPO " ./" > /etc/apt/sources.list.d/osmocom-latest.list
 
+ADD	$OSMOCOM_REPO/Release /tmp/Release
 RUN	apt-get update && apt-get install -y \
 	eclipse-titan