I have been looking for a Docker image of FreeBSD but cannot find, can FreeBSD be run inside docker? If not, why not?
Technically yes, but you need a FreeBSD host to do it and Docker is "somewhat unstable" on FreeBSD right now. There is a fork from Docker 1.7 that can technically launch containers, but nothing you would want to use for reals. Jetpack will hopefully be finalized for FreeBSD 11 and will provide a much better solution here.
https://wiki.freebsd.org/Docker has info. You can make it work on a fully updated 10.3 but expect lots of weirdness.