Configure Jenkins to use debian-jessie based buck-slave

Since change 184713 the docker image of the buck-slave is based
on debian jessie. The image was tagged 'debian-jessie', but the
configuration-file of Jenkins was not updated to use the updated
image.

This change updates the configuration accordingly.

Change-Id: I1297780ebeaed8c739727ddbce36207db6ccd0cf
diff --git a/jenkins-docker/master/config.xml b/jenkins-docker/master/config.xml
index 08ea2b3..c756d99 100644
--- a/jenkins-docker/master/config.xml
+++ b/jenkins-docker/master/config.xml
@@ -58,7 +58,7 @@
           </retentionStrategy>
           <numExecutors>1</numExecutors>
           <dockerTemplateBase>
-            <image>gerritforge/gerrit-ci-slave-buck</image>
+            <image>gerritforge/gerrit-ci-slave-buck:debian-jessie</image>
             <dockerCommand></dockerCommand>
             <lxcConfString></lxcConfString>
             <hostname></hostname>