Currently container guidelines[1] have dnf clean in the template but they don't have it as a requirement. Due to size effect dnf cache has on containers I believe it should be required to clean. Maybe word it as a general requirement of:

Dockerfile MUST clean temporary files created within the container image during the build process to keep the size of container images small.

Or something of the sort

[1] https://fedoraproject.org/wiki/Container:Guidelines