Set the hostname of a Docker Container same as that of your host machine

Setting Container Hostname to be same as your Host Machine

Sounds trivial, but it turns out you need to do some tweaks in order to set the hostname
to be the same as that of Host Machine.

Scenario

I had a requirement at work, where I needed to set the hostname of a specific container
to be that of the Machine it was supposed to run on.

The Linux Distribution under consideration was Ubuntu 20.04 LTS.

Ubuntu's mysterious HOST and HOSTNAME environment variables

If you are on an Ubuntu machine right now, try echo $HOS and press the TAB key to let the bash completion fill out. High chances that you will see HOST and HOSTNAME
as available Environment Variables already available to your bash shell's session.

Fairly Simple, you could simply use either one of them in your Compose file as an environment variable to the hostname key and should work out of the box! Not quite!

Investigation via a simple Example

Take the following docker-compose.yml file

services:
  test:
    image: alpine:latest
    container_name: hostname-tester
    hostname: alpine-${HOST}
    command:
      - hostname

The following test service should be able to retrieve the HOST variable value and set it as the hostname of the alpine container. The command should be able to print something like alpine-my-ubuntu as an example.

Let's see what happens when we run docker compose up

WARN[0000] The "HOST" variable is not set. Defaulting to a blank string. 
[+] Running 1/1
 ⠿ Container hostname-tester  Recreated                                    0.1s
Attaching to hostname-tester
hostname-tester  | alpine-
hostname-tester exited with code 0

As you see the HOST variable although available to the shell is not available within the
Compose file.

Most of the shell environment variables are visible via using printenv on the host machine, so a quick search for HOST or HOSTNAME reveals that these specific environment variables are not in the env of the shell

printenv | grep -i "host"

Solution

The most standard way to make a variable available for a shell session is by exporting it.

Let's export HOST using:

export HOST

Let's try bringing the container back up again and see if it picks up the variable values

❯ export HOST
❯ docker compose up
[+] Running 1/0
 ⠿ Container hostname-tester  Recreated                                    0.0s
Attaching to hostname-tester
hostname-tester  | alpine-shan-pc
hostname-tester exited with code 0

for my machine (Manjaro Linux (Rolling)) I could now set the hostname of the container to be the same as that of the Host Machine.

Sure enough searching through printenv again you will find the export HOST variable.

If you have on-prem servers or cloud images where you might need such a configuration a solution would be to add export HOST to your user's ~/.bashrc or ~/.zshrc (if using ZSH)
and the value will be available when bringing the corresponding Compose Stack up.

Hope this information helps people trying to find a similar solutions when it comes to Docker and Docker Compose based environments.