Containers (and virtualisation) are helpful but. not bulletproof. If your threat model is already malicious code with ill intent, you probably want higher grades of privsep / sandboxing.
Docker has had it's fair share of breakouts because the default configuration doesn't use user namespaces (one of the most significant security isolation features in Linux) and runs as root. It's not indicative of how secure a properly set up container is -- I don't remember the last time there was an LXC or LXD breakout (which use user namespaces by default).
Source: I've found a fair few Docker breakouts. I also maintain runc, which is the lower-level container runtime component (where most of the breakouts are found).
There have been security bugs involving allowing unprivileged user namespaces, but that doesn't matter at all in this case -- seccomp is used by effectively all container runtimes to block things like CLONE_NEWUSER inside containers.
E.g.,
https://www.twistlock.com/labs-blog/escaping-docker-containe...
https://www.exploit-db.com/exploits/46978
Full virtualisation does far better, though I believe there've been exploits there as well. Yes, from 2015:
https://threatpost.com/xen-patches-7-year-old-vm-escape-hype...