After I ran the dokku installation script on my fresh new Ubuntu system, the console outputed such error:

This installation script requires that you have a hostname set for the instance. Please set a hostname for 127.0.0.1 in your /etc/hosts

The solution, indeed, is fairly simple. You only need to add the following lines to the /etc/hosts and /etc/hostname files:

// This line goes to /etc/hosts
YOUR_IP YOUR_URL YOUR_HOSTNAME

// This line must be included in your /etc/hostname file
YOUR_HOSTNAME

Then, you can test it by hostname -f.