fix git checkout

docker/Dockerfile.go_build

@@ -4,7 +4,7 @@ RUN mkdir -p /go/src/github.com/seaweedfs/
 RUN git clone https://github.com/seaweedfs/seaweedfs /go/src/github.com/seaweedfs/seaweedfs
 ARG BRANCH=${BRANCH:-master}
 ARG TAGS
-RUN cd /go/src/github.com/seaweedfs/seaweedfs && git checkout $BRANCH
+RUN cd /go/src/github.com/seaweedfs/seaweedfs && (git checkout $BRANCH || (git fetch --unshallow && git checkout $BRANCH))
 RUN cd /go/src/github.com/seaweedfs/seaweedfs/weed \
   && export LDFLAGS="-X github.com/seaweedfs/seaweedfs/weed/util/version.COMMIT=$(git rev-parse --short HEAD)" \
   && CGO_ENABLED=0 go install -tags "$TAGS" -ldflags "-extldflags -static ${LDFLAGS}"