Use Debian for Buck build image

The new Debian image should be used for all builds
including Buck.

The old Ubuntu 15.x is now obsolete and can be safely
discontinued. Will do that in the follow-up change when
no more images will point to it.

Change-Id: I5c0401a3db1ce407841fb297bff85072204914d7
diff --git a/jenkins-docker/slave-buck/Dockerfile b/jenkins-docker/slave-buck/Dockerfile
index 1d03be0..db49a4a 100644
--- a/jenkins-docker/slave-buck/Dockerfile
+++ b/jenkins-docker/slave-buck/Dockerfile
@@ -1,4 +1,4 @@
-FROM gerritforge/gerrit-ci-slave
+FROM gerritforge/gerrit-ci-slave-debian:jessie
 
 # Buck build
 
diff --git a/jenkins-docker/slave-buck/Makefile b/jenkins-docker/slave-buck/Makefile
index 30fb320..aa87d88 100644
--- a/jenkins-docker/slave-buck/Makefile
+++ b/jenkins-docker/slave-buck/Makefile
@@ -1,5 +1,5 @@
 NO_CACHE=false
-IMAGE=gerritforge/gerrit-ci-slave-buck
+IMAGE=gerritforge/gerrit-ci-slave-buck:debian-jessie
 
 # Targets