Force Docker onto overlay2

Both btfs and overlay file systems suffer a very strange bug when executing the following Dockerfile:

```
FROM alpine

RUN mkdir lch

COPY . lch/
COPY requirements.txt lch/requirements.txt
COPY ./requirements/ lch/requirements/

ENTRYPOINT ["/docker-entrypoint.sh"]
```

It fails on the last `COPY` line, due to the presence of the `COPY . lch/` call. Unknown as to why, but moving to the new filesystem fixes things.

buildman/templates/cloudconfig.yaml

@@ -19,6 +19,13 @@ ssh_authorized_keys:
 - ssh-rsa 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 EvB Key
 
 write_files:
+- path: /etc/docker/daemon.json
+  permission: '0644'
+  content: |
+    {
+        "storage-driver": "overlay2"
+    }
+
 - path: /root/overrides.list
   permission: '0644'
   content: |
@@ -69,30 +76,6 @@ 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={{ volume_size }} /var/lib/docker.btrfs
-        ExecStart=/usr/sbin/mkfs.btrfs /var/lib/docker.btrfs
-    - name: var-lib-docker.mount
-      enable: true
-      content: |
-        [Unit]
-        Before=docker.service
-        After=format-var-lib-docker.service
-        Requires=format-var-lib-docker.service
-        [Install]
-        RequiredBy=docker.service
-        [Mount]
-        What=/var/lib/docker.btrfs
-        Where=/var/lib/docker
-        Type=btrfs
-        Options=loop,discard
     - name: machine-lifetime.service
       command: start
       enable: yes