Containers provide a level of isolation relative to standard Linux processes. Docker style containers were not designed specifically as a security sandbox (IMO)

Containers provide a level of isolation relative to standard Linux processes. Docker style containers were not designed specifically as a security sandbox (IMO) but instead re-use a set of Linux primitives to provide a level of isolation. One of the good parts about this is that as they use Linux primitives, the level of isolation can be increased relatively easily. Things like --cap-drop=ALL and --no-new-privileges, combined with ensuring the contained process(es) don't run as root can make things better. It's also important to ensure that the isolation put in place isn't removed. The major case where this happens is that Kubernetes disables Docker's seccomp filter which has left containers running in k8s clusters vulnerable to issues that stock Docker containers were not (CVE-2022-0185 for a recent example). If I was running truly untrusted processes from unknown people, I probably would not use Docker containers, instead I'd use gVisor or Firecracker.