bug(docker): bind docker bridge to a specific IP

Docker chooses an unused IP range for its network bridge, but more
than once we have seen this conflict with the Vagrant bridge, which
is presumably created after the Docker bridge. This commit uses
a specific CIDR for the Docker bridge, which does not conflict with the
Vagrant IP.

fixes #702

Author: carmstrong

contrib/coreos/user-data

@@ -15,7 +15,7 @@ coreos:
       ExecStartPre=/bin/mount --make-rprivate /
       # Run docker but don't have docker automatically restart
       # containers. This is a job for systemd and unit files.
-      ExecStart=/usr/bin/docker -d -s=btrfs -r=false -H fd://
+      ExecStart=/usr/bin/docker -d -s=btrfs --bip=10.212.0.0/16 -r=false -H fd://
 
       [Install]
       WantedBy=multi-user.target