Add jvm gc logging to gerrit and traceability perms

We are seeing java gc go crazy at times and aren't quite sure what is
causing it. Add jvm gc logging to the gerrit process to help us identify
what is happening.

Additionally we add SYS_PTRACE to the container capabilities so that you
can get heap dumps from the jvm. To get a heap dump you need to do
roughyl:

  docker exec -u root -it gerritcontainerid jhsdb jmap --heap --pid $pid

Change-Id: Ib4a5b84fda4eca73c7971c31ee74c3232eb733e4

playbooks/roles/gerrit/templates/docker-compose.yaml.j2

@@ -11,13 +11,17 @@ services:
 {% for volume in gerrit_container_volumes %}
       - {{ volume }}
 {% endfor %}
-{% if gerrit_heap_limit is defined %}
     environment:
-      JAVA_OPTIONS: "-Xmx{{ gerrit_heap_limit }}"
+      JAVA_OPTIONS: >-
+        -Xlog:gc*:file=/var/gerrit/logs/jvm_gc.log:time,uptime,tid,level,tags:filecount=10,filesize=20M
+{% if gerrit_heap_limit is defined %}
+        -Xmx{{ gerrit_heap_limit }}
 {% endif %}
     ulimits:
       core: 0
       nofile: {{ gerrit_packed_git_open_files * 2 }}
+    cap_add:
+      - SYS_PTRACE
   # Utility "service" to allow us to run ad-hoc commands
   shell:
     image: {{ gerrit_container_image }}