feat(app-emulation/docker): add docker

add docker and lxc
fix(sys-kernel/coreos-kernel): use kernelrelease

kernelversion is not used for the binary file name. Use kernelrelease.
This fixes an issue where the /boot/vmlinuz symlink wasn't pointing at
the right file causing builds to fail.
fix(app-emulation/docker): requires brctl

http://stackoverflow.com/questions/15768121/docker-net-no-such-interface
feat(coreos-base/coreos): install docker by default