Boot2docker runs the /var/lib/boot2docker/bootsync.sh script when booting. You can use this file to setup a static ip address and default gateway during boot:

$ cat <<EOF | docker-machine ssh docker-host sudo tee /var/lib/boot2docker/bootsync.sh > /dev/null 
ifconfig eth0 10.0.0.10 netmask 255.255.255.0 broadcast 10.0.0.255 up
ip route add default via 10.0.0.1
EOF

After modifying this file, restart the host, verify its settings and regenerate client-certificates

$ docker-machine restart docker-host
$ docker-machine env docker-host
$ docker-machine regenerate-certs docker-host