Set GOPATH and permissions for buildifier

Change-Id: I43fc8ffece383f5952d61570a2f690ef2705e96f
diff --git a/jenkins-docker/slave-bazel/Dockerfile b/jenkins-docker/slave-bazel/Dockerfile
index 6f0706f..ac14329 100644
--- a/jenkins-docker/slave-bazel/Dockerfile
+++ b/jenkins-docker/slave-bazel/Dockerfile
@@ -13,7 +13,8 @@
     tar xvfz /tmp/golang.tar.gz && \
     cd /usr/local/bin && \
     ln -s /usr/local/go/bin/* . && \
-    go get github.com/bazelbuild/buildtools/buildifier
+    GOPATH=/usr go get github.com/bazelbuild/buildtools/buildifier && \
+    chmod a+rx /usr/bin/buildifier
 
 # Bazel cache warm-up with Gerrit master build
 # Set-up google-java-format utility to ~/format/google-java-format