Is it safe to use root user in some containers?

I know that from a safety point of stand, root access can be a vulnerability especially in the case of uninspected third party containers, but I'm a bit confused about the security perspective of containers.

If the containerization solves the security problem by logical separation of these units, does that mean that a root user in one container can do no harm to other containers, and the underlying system?

I came across this problem because I'm trying to deploy a test app in a kubernetes/rancher system, and it uses a php-appache container, however upon deploying, since the base image wants to use 80 port for the apache, and I set a simple user for the docker, the system throws an error that the socket cannot be made (I know this is because ports below 1024 are exclusively for the root) however the base image does not contain any configuration setting to change the default port in a simple way, so I had to tinker.

And I started wondering, if the base image did not have any way of setting a different port than 80, that implies that the image should run with a root user?