osmo-mgw-master: add Dockerfile ARG for git repo branch to be built

This allows the user to build any non-master branches of the related
repositories, which is helpful for local testing before pushing changes
to git.

Change-Id: I42054d585a5774452a13750ee1c3e6bb8013b26f

osmo-mgw-master/Dockerfile

@@ -22,11 +22,14 @@ RUN	apt-get update && \
 
 WORKDIR	/tmp
 
+ARG	OSMO_MGW_BRANCH="master"
+
 RUN	git clone git://git.osmocom.org/osmo-mgw.git
-ADD	http://git.osmocom.org/osmo-mgw/patch /tmp/commit-osmo-mgw
+ADD	http://git.osmocom.org/osmo-mgw/patch?h=$OSMO_MGW_BRANCH /tmp/commit-osmo-mgw
+
 
 RUN	cd osmo-mgw && \
-	git fetch && git checkout -f -B master origin/master && \
+	git fetch && git checkout -f -B $OSMO_MGW_BRANCH origin/$OSMO_MGW_BRANCH && \
 	autoreconf -fi && \
 	./configure && \
 	make -j8 install && \