Use Gerrit 3.1.15 in Dockerfile

Now that 3.1 is EOL (3.4 released), set GERRIT_CI_URL to the archive.

Change-Id: I984bfb5020b6fe0a2215d9f690d563942b3c3eaf

src/test/docker/gerrit/Dockerfile

diff --git a/src/test/docker/gerrit/Dockerfile b/src/test/docker/gerrit/Dockerfile
index 5f455d4..c40d91b 100644
--- a/src/test/docker/gerrit/Dockerfile
+++ b/src/test/docker/gerrit/Dockerfile
@@ -1,8 +1,8 @@
-FROM gerritcodereview/gerrit:3.1.12
+FROM gerritcodereview/gerrit:3.1.15
 
 ENV GERRIT_BRANCH=stable-3.1
 
-ENV GERRIT_CI_URL=https://gerrit-ci.gerritforge.com/job
+ENV GERRIT_CI_URL=https://archive-ci.gerritforge.com/job
 
 USER root