Many articles mention that there is one more way to run a container as a different user: using docker-compose, a tool that instruments Docker on how to run our containers. In short: instead of command line parameters, we use a structured config file that can look like this:

version: '3'
 services:
  my_service:
     image: my_image 
     command: /bin/bash

We can start our service by running:

$ docker-compose run my_service

Which is equivalent to:

$ docker run -it my_image /bin/bash

Specifying a UID is just a one more line in the config file:

version: '3'
 services:
  my_service:
     image: my_image
     user: $UID:$GID
     command: /bin/bash

Unfortunately, bash does not set GID by default, so we need to do it before running docker-compose. Using the id command inside a config file won’t work as the file is not pre-processed in any way.

$ GID=$(id -g) docker-compose run my_service
$$ id
uid=1000 gid=1000 groups=1000

Conclusion

Docker-compose is a nice tool that wraps docker run command and allows to make a configuration part of the project. It can be useful when creating build environments, automated tests, or complex run configurations.