Docker is just namespaces and cgroups. You do not have separation between host and the containers on Linux - you need additional software for that - like AppArmor or SELinux. The FreeBSD Jails are secure and separated from the host on the contrary. FreeBSD Host can access Jails but Jails can not access Host resources - unless consciously configured to do so.

As for the usage ... FreeBSD Jails can have their OWN network stack (with own firewall and interfaces) while Docker can only use Linux network namespaces.

You can launch/start a single process Jail and a single process Docker - here they are similar. You can also unpack entire operating system userland in both and it will also run - with own packages database etc.

Hope that helps.