Adding a local system variable, or the result of a command, to a dockerfile

Question

I have seen some similar questions, but none of them appear to solve my problem. I want to add a user to a docker container and in my Dockerfile, I define the username with:

ARG USERNAME="some_user"

Instead, I want the username to be the current user's computer username, as obtained by running the command whoami in the local terminal.

So what I would like to have is something like

ARG USERNAME=$(whoami) .

This $(whoami) should be obtained from the local system environment, and not from the docker container. Is there a way to do this for dockerfiles? I have thought of .env and docker-compose solutions but these also require each user to set their own username according to my knowledge.

Answer 1

There is no integrated way to execute arbitrary commands on the host directly outside of a container using just docker build / docker-compose build .

So to execute an arbitrary command to get/generate the required information you'll need to provide a custom script / use another build system to call docker / docker-compose with the respective flags or maybe generate the .env file from a template / interactively.

If you only need the current user name you may want to use the $USER / $LOGNAME environment variables that are set by the system in many default configurations. But since these are just normal environment variables their values may be incorrect / empty / manually changed by the user, see this question .