Avoid cloning buck twice in jenkins-docker/slave-buck

During the build of jenkins-docker/slave-buck buck was cloned twice.
Due to the size of the project, this took quite long.

This change duplicates the local repository by copying before
building buck, which should be faster than cloning it again.

Change-Id: I354ea5b9452d9bca5ba53a42c34f2c4eeead5a73
diff --git a/jenkins-docker/slave-buck/Dockerfile b/jenkins-docker/slave-buck/Dockerfile
index a3f3942..7555d9c 100644
--- a/jenkins-docker/slave-buck/Dockerfile
+++ b/jenkins-docker/slave-buck/Dockerfile
@@ -3,10 +3,9 @@
 # Buck build
 
 RUN git clone https://github.com/facebook/buck /opt/buck-java8 && \
+    cp -r /opt/buck-java8 /opt/buck-java7 && \
     cd /opt/buck-java8 && ant && \
-    chown -R jenkins:jenkins /opt/buck-java8
-
-RUN git clone https://github.com/facebook/buck /opt/buck-java7 && \
+    chown -R jenkins:jenkins /opt/buck-java8 && \
     cd /opt/buck-java7 && git checkout 6a42de3b4b7989d9c042c5524d765bf4a462dcb8 && \
     bash -c '. /usr/bin/set-java.sh 7 && ant' && \
     chown -R jenkins:jenkins /opt/buck-java7