Get environment value in controller

In my .env file I have the following:

IMAP_HOSTNAME_TEST=imap.gmail.comIMAP_USERNAME_TEST=myemail@gmail.comIMAP_PASSWORD_TEST=mypw

Now I would like to use them in my controller. I've tried this, but without any result:

$hostname = config('IMAP_HOSTNAME_TEST');

The $hostname variable is equal to null. How can I use these configuration variables in my controller?