If you have a Docker image created with a non-root user using USER in your Dockerfile, but you need to su to root to install or update something owned by root, without setting a root password you won’t be able to su to root.
Instead, add a password for root in your Dockerfile (this is described here):
RUN echo "root:Docker!" | chpasswd
This is probably not a good idea for security reasons (especially if you are sharing your Dockerfile), but I needed to create something in a container to reuse when creating other new containers, so did this one-time to get the file I needed and then reused the file in containers with another image without the root user/password.