tl;dr: specify the --user root argument:

docker exec --interactive \
            --tty \
            --user root \
            --workdir / \
            container-name bash

There are good reasons why running in a container as root is not a good idea, and that’s why many images published nowadays avoid doing this. Confluent Platform’s Docker images changed to using appuser with the 6.0 release.

Checking the container user ðŸ”—

You can check what user your container is running with:

$ docker exec --interactive --tty kafka bash
[appuser@b59043522a44 ~]$ whoami
appuser

or more directly:

$ docker exec --interactive --tty kafka whoami
appuser

Changing the container user ðŸ”—

Using the --user root argument when launching the Docker exec command you can override the container’s user:

$ docker exec --interactive --tty --user root kafka bash
[root@b59043522a44 appuser]# whoami
root

or

$ docker exec --interactive --tty --user root kafka whoami
root

What, no sudo? ðŸ”—

Imagine this nightmare scenario 🙀 :

$ docker exec --interactive --tty kafka bash
[appuser@b59043522a44 ~]$ yum install jq
Error: This command has to be run under the root user.
[appuser@b59043522a44 ~]$ sudo yum install jq
bash: sudo: command not found
[appuser@b59043522a44 ~]$

Now, installing into Docker containers is not The Right Way - you should amend the Docker image to install what’s needed before invocation as a container. BUT sometimes needs must. Whether a quick hack, or just a PoC that you want to get running - sometimes you do want to install into a container, and that can be more difficult without root.

You can use the same approach as above (--user root):

$ docker exec --interactive --tty --user root kafka bash
[root@b59043522a44 appuser]# yum install -y jq
Confluent repository                                                                                                                                         13 kB/s |  29 kB     00:02
Red Hat Universal Base Image 8 (RPMs) - BaseOS                                                                                                              978 kB/s | 772 kB     00:00
Red Hat Universal Base Image 8 (RPMs) - AppStream                                                                                                           1.8 MB/s | 4.9 MB     00:02
Red Hat Universal Base Image 8 (RPMs) - CodeReady Builder                                                                                                    40 kB/s |  13 kB     00:00
zulu-openjdk - Azul Systems Inc., Zulu packages                                                                                                              95 kB/s | 123 kB     00:01
[…]
]
Installed:
  jq-1.5-12.el8.x86_64                                                                     oniguruma-6.8.2-2.el8.x86_64

Complete!

Logging in as root on Oracle’s Database Docker Image ðŸ”—

Using Oracle’s Docker database image I wanted to install some additional apps, without modifying the Dockerfile.

Connect to the container:

$ docker exec --interactive --tty docker-compose_oracle_1_479e7fa05ab5 bash

No sudo:

[oracle@a37d6e99353b ~]$ sudo whoami
bash: sudo: command not found

Googled, found the the --user flag for Docker, tried that:

$ docker exec --interactive --tty --user root docker-compose_oracle_1_479e7fa05ab5 bash
OCI runtime exec failed: exec failed: container_linux.go:348: starting container process caused "chdir to cwd (\"/home/oracle\") set in config.json failed: permission denied": unknown

Evidently, the Docker image tries to change directory to the Oracle home folder which Docker’s not happy doing as another user (even though it’s root?).

Googled some more, found the --workdir flag to override the WORKDIR setting of the Dockerfile from which the image is built:

$ docker exec --interactive --tty --user root --workdir / docker-compose_oracle_1_479e7fa05ab5 bash
bash-4.2# whoami
root