Docker containers aren't provably secure. If you want isolation, use a VM that doesn't have host file system access. This way, if the VM is compromised, just throw it away and it can't leak out the way containers do.
Not only are they not provably secure (very few things are), they are explicitly not intended for use as a security boundary. Their whole gimmick is lightweight containers you can use instead of VMs if you trust everyone who's going to run code under them.
To disambiguate: I don't mean formal verification like seL4, I mean it hasn't been thoroughly audited to show it is reasonably secure. Docker security of images and running containers is pretty shit as I brought up on GH in the beginning. Developers just shrugged it off and focused on whiz-bang features.
The conflation of what amounts to fancy Linux cgroups trickery with hypervisors is a depressing misunderstanding of isolation.