Which user runs the process inside the Docker container?

By default, the process inside a Docker container runs as the root user (UID 0). This is because containers are isolated, and there is no need for additional privileges inside them as on the host system.

However, running processes as root inside a container can be unsafe. Therefore, it is common to specify a different user in the Dockerfile or when running the container using the USER directive, for example:

USER appuser

Or when starting the container:

docker run -u 1000 myimage

Thus, the user under which the process runs depends on the container settings, but by default, it is root.