Makes dockerui compatible with dokku

These changes allow you to deploy dockerui to dokku.

All these changes are really to make the go buildpack happy.

Once pushed to dokku, configure docker to bind on the docker0 bridge :

```
ip a s docker0 | grep inet
=> inet 172.17.42.1/16 scope global docker0
echo 'DOCKER_OPTS="-H unix:///var/run/docker.sock -H tcp://172.17.42.1:4243 -api-enable-cors"' > /etc/default/docker
restart docker
dokku config:add dockerui DOCKER_ENDPOINT=http://172.17.42.1:4243
```