Secure for what? I mean, sandboxing is more or less inherently a security technique. An app running in a Docker (or whatever) container is less exposed than one running on a big shared host. Is it as firm an isolation as you get from a proper virtual machine layer? No. But then, as we've discovered over the past year the hardware itself can have bugs that poke holes in that protection.

No free lunch. But yeah, Linux's suite of "container" tools are basically as secure as any other OS's "containers", no matter what spin you're reading elsewhere.