$(uname -a) returning the same in docker host or any docker container

Question

Why $(uname -a) returns the same Linux kernel than the host machine inside a docker container?

uname -a #=> Linux leo 3.13.0-55-generic #94-Ubuntu SMP Thu Jun 18 00:27:10 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux  docker run -ti --rm ubuntu:vivid-20150611 uname -a #=> Linux 147c3aff216b 3.13.0-55-generic #94-Ubuntu SMP Thu Jun 18 00:27:10 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux 

Even though my host machine is Ubuntu 14.04 (trusty) while the container is Ubuntu 15.04 (vivid) and should have kernel 3.19