How can you change the user inside a Dockerfile during container build?

In a Dockerfile, to change the user under which subsequent commands are run or the container is executed, the USER instruction is used.

Example:

FROM ubuntu:20.04

# Create a user and group
RUN groupadd -r myuser && useradd -r -g myuser myuser

# Change the user
USER myuser

# Subsequent commands will run as myuser
CMD ["bash"]

If you need to switch to a user with a specific UID/GID, you can specify them directly:

USER 1001:1001

It is important that the user exists in the container, otherwise the command will result in an error.