Manually apply MAC address when supplied (fixes #1)

If specified (via `--mac-address`), Docker only applies a user-selected
MAC address when moving the interface into the container's network
namespace. This means that the initial run of `udhcpc` will use the
random veth MAC address, breaking any statically configured leases
(since this is the IP address that Docker will assign to the container).

net-dhcp/network.py

@@ -161,7 +161,11 @@ def create_endpoint():
     }
 
     try:
-        if 'MacAddress' not in req_iface or not req_iface['MacAddress']:
+        if 'MacAddress' in req_iface and req_iface['MacAddress']:
+            (if_container
+            .set('address', req_iface['MacAddress'])
+            .commit())
+        else:
             res_iface['MacAddress'] = if_container['address']
 
         def try_addr(type_):