Fix cache warm-up in jenkins-docker/slave-buck

The slave-buck image performed the cache warm-up with the gerrit
version on master branch, which is by now using bazel to build.
Thus the build failed.

This change changes the Dockerfile to provide a working buck version
and to perform the cache warm-up using the stable-2.13 branch.

Change-Id: Ic19ae4db9b1384c51ae4cb5469c9c8eb7a5c48f0
diff --git a/jenkins-docker/slave-buck/Dockerfile b/jenkins-docker/slave-buck/Dockerfile
index 7555d9c..1d03be0 100644
--- a/jenkins-docker/slave-buck/Dockerfile
+++ b/jenkins-docker/slave-buck/Dockerfile
@@ -4,8 +4,8 @@
 
 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 && \
+    cd /opt/buck-java8 && git checkout e64a2e2ada022f81e42be750b774024469551398 && \
+    ant && 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
@@ -34,11 +34,11 @@
 # Buck JVM options setup
 COPY buckjavaargs /home/jenkins/.buckjavaargs
 
-# Buck cache warm-up with Gerrit master build
+# Buck cache warm-up with Gerrit 2.13 build
 USER jenkins
-RUN bash -c '. /usr/bin/set-java.sh 8 && \
-    cd /tmp &&  git clone --recursive https://gerrit.googlesource.com/gerrit && cd /tmp/gerrit && \
-    buck build gerrit api plugins:core || true  && \
+RUN bash -c '. /usr/bin/set-java.sh 8 && cd /tmp &&  \
+    git clone -b stable-2.13 --recursive https://gerrit.googlesource.com/gerrit && \
+    cd gerrit && buck build gerrit api plugins:core || true  && \
     buck test --dry-run --no-results-cache --exclude flaky || true  && \
     rm -Rf /tmp/gerrit /home/jenkins/.gerritcodereview/buck-cache/{cache,locally-built-artifacts}'