Detect if `c mk docker-env` has been run.

[allanchau]

We should warn the user if c mk docker-env hasn't been run yet.

Something like this could be added as part of the checks the cli does before running a command. Alternatively we could automate it and run c mk docker-env instead of displaying an error?

if (!process.env.DOCKER_HOST) {
    console.error(chalk.red('\nCould not find $DOCKER_HOST, have you run "c mk docker-env" yet?'));
}