docs: --user is only required when docker run as root

The --user argument attempts to map the uid of the user inside the
container to the user in the host. This works if docker is running as
root, but is docker is running as the current user, then the uid in the
container is mapped to a surrogate uid on the host, and this surrogate
user does not have permissions to complete the build process.

Clarify that the --user flag is only required when running docker as
root. It is also likely not required by users using podman as a docker
drop-in replacement, since podman always runs in rootless mode.

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