How does Docker allow portable containers if the Kernel libraries change
There is no such thing as a "kernel library". The closest things to what you're describing are:
libc, which is part of the container image, and hence does not change.The Linux kernel ABI, which is mostly constant. While some changes are occasionally made to the kernel ABI, this is done as rarely as possible - kernel developers do everything possible to maintain backwards compatibility. Where changes are made, it is most often in components that would not be relevant to applications running in a container (e.g, audio / video output, dynamic device management, etc).