Dockerfile needs branch setting

Future releases need to adjust the branch where the image is built from.

extras/docker/unstable/Dockerfile

@@ -9,16 +9,16 @@ LABEL description="Development build"
 ENV BUILD_HOME=/build
 ENV GOPATH=$BUILD_HOME/golang
 ENV PATH=$GOPATH/bin:$PATH
-ENV HEKETIC_CONF_DIR=/etc/heketi
+ENV HEKETI_BRANCH="master"
 
 # install dependencies, build and cleanup
-RUN mkdir $BUILD_HOME $GOPATH $HEKETI_CONF_DIR && \
+RUN mkdir $BUILD_HOME $GOPATH && \
     dnf -q -y install golang git && \
     dnf -q -y install make && \
     dnf -q -y clean all && \
     mkdir -p $GOPATH/src/github.com/heketi && \
     cd $GOPATH/src/github.com/heketi && \
-    git clone https://github.com/heketi/heketi.git && \
+    git clone -b $HEKETI_BRANCH https://github.com/heketi/heketi.git && \
     go get github.com/tools/godep && \
     cd $GOPATH/src/github.com/heketi/heketi && make && \
     cp heketi /usr/bin/heketi && \