buildman: create and mount btrfs volume for docker

There are numerous issues with overlayfs that actually aren't present with
btrfs. Btrfs seems to have long-running issues, but our builders are
ephemeral. Example issue: https://github.com/docker/docker/issues/10180

buildman/templates/cloudconfig.yaml

@@ -50,3 +50,25 @@ coreos:
                      after_units=['quay-builder.service']
                      ) | indent(4) }}
     {%- endif %}
+    - name: format-var-lib-docker.service
+      command: start
+      content: |
+        [Unit]
+        Before=docker.service var-lib-docker.mount
+        ConditionPathExists=!/var/lib/docker.btrfs
+        [Service]
+        Type=oneshot
+        ExecStart=/usr/bin/truncate --size=28G /var/lib/docker.btrfs
+        ExecStart=/usr/sbin/mkfs.btrfs /var/lib/docker.btrfs
+    - name: var-lib-docker.mount
+      command: start
+      content: |
+        [Unit]
+        Before=docker.service
+        After=format-var-lib-docker.service
+        Requires=format-var-lib-docker.service
+        [Mount]
+        What=/var/lib/docker.btrfs
+        Where=/var/lib/docker
+        Type=btrfs
+        Options=loop,discard