Dockerfile: build with the RADOS driver

Signed-off-by: Vincent Giersch <vincent.giersch@ovh.net>

Dockerfile

@@ -1,7 +1,12 @@
 FROM golang:1.4
 
+RUN apt-get update && \
+    apt-get install -y librados-dev && \
+    rm -rf /var/lib/apt/lists/*
+
 ENV DISTRIBUTION_DIR /go/src/github.com/docker/distribution
 ENV GOPATH $DISTRIBUTION_DIR/Godeps/_workspace:$GOPATH
+ENV DOCKER_BUILDTAGS include_rados
 
 WORKDIR $DISTRIBUTION_DIR
 COPY . $DISTRIBUTION_DIR