User created in Dockerfile with multiple groups does not have additional groups inside the docker image

General
1

Hi Docker Gurus,

I have seeing a weird problem when creating a docker image. I have the following RUN instruction in my Dockerfile:

RUN addgroup -S ubuntu && \
    adduser -S -D ubuntu -h /home/ubuntu -s /bin/bash -G ubuntu && \
    addgroup ubuntu root

But when I use the docker image in a Concourse CICD, the user ubuntu has only one group:

bash-5.0$ id
uid=101(ubuntu) gid=102(ubuntu) groups=102(ubuntu)

But in the container, when I use su command to login as ubuntu again, all the groups are as expected for ubuntu.

bash-5.0$ sudo su - ubuntu
ACMv1-: id
uid=101(ubuntu) gid=102(ubuntu) groups=102(ubuntu),0(root)

I expected the root and ubuntu groups to be associated with the user ubuntu even before trying su command. Appreciate any help in this matter.

2

It would be important to know the base image. I guess it is alpine, since addgroup works differently in a standard ubuntu image. If I am right, it is a little weird to create an ubuntu user in an Alpine image, although I am sure you have a good reason.

Without the base image, I can only share one of my old experiences. The only time I had a similar problem (not in containers) when I installed a knew Linux distribution, probably Kubuntu and an authentication module (PAM) caused that I could not give sudo permission to my user until I disabled that module. I don’t remember the details…