When you introduce a new environment variable in bash:

export DOCKER_HOST=localhost:2375

It will disappear after you restart your terminal, reconnect via ssh, whatever.

In order not to miss the variable or your setting of choice later on, just append the appropriate command to the .bash_profile file:

echo "export DOCKER_HOST=localhost:2375" >> ~/.bash_profile

echo "alias docker='sudo docker'" > .bash_profile