docker-for-mac: supply the daemon.json via metadata

Previously the docker-for-mac example had a `files` declaration
which wrote `/var/config/docker/daemon.json`. This was probably an
error because the `/var/config` directory is owned by the `metadata`
package. It used to work -- probably by accident -- but doesn't
any more.

This patch switches to using the metadata package to populate the
`daemon.json`.

Signed-off-by: David Scott <dave.scott@docker.com>

blueprints/README.md

@@ -33,7 +33,7 @@ $ moby build -name docker-for-mac base.yml docker-ce.yml
 To run the VM with a 500M disk:
 
 ```
-linuxkit run hyperkit -networking=vpnkit -vsock-ports=2376 -disk size=500M docker-for-mac
+linuxkit run hyperkit -networking=vpnkit -vsock-ports=2376 -disk size=500M -data ./metadata.json docker-for-mac
 ```
 
 In another terminal you should now be able to access docker via the socket `guest.00000947` in the state directory (`docker-for-mac-state/` by default):

blueprints/docker-for-mac/docker-ce.yml

@@ -23,11 +23,6 @@ services:
             "--swarm-default-advertise-addr=eth0",
             "--userland-proxy-path", "/usr/bin/vpnkit-expose-port",
             "--storage-driver", "overlay2" ]
-
-files:
-    - path: /var/config/docker/daemon.json
-      contents: '{ "debug": true }'
-
 trust:
     org:
         - library

blueprints/docker-for-mac/metadata.json

@@ -0,0 +1,8 @@
+{
+  "docker": {
+    "daemon.json": {
+      "perm": "0644",
+      "content": "{ \"debug\": true }"
+    }
+  }
+}