I have changed
DOCKER_OPTS="-H tcp://127.0.0.1:2375 -H unix:///var/run/docker.sock" (docker version 1.4.1 in ubuntu 14.04), but it do not take any effect for me (not listening at port
2375). It seems that docker do not read this initial config file because I found
export http_proxy enviroment do not work too.
sudo docker -H tcp://127.0.0.1:2375 -H unix:///var/run/docker.sock -d works.
It really confused me!