Slice units only help with resource allocation, but the main draw of Docker isn't that, but the fact that you can be running a completely different distro with a completely different stack since a container is basically a glorified chroot and that the image can be built from an easy to understand script and then shared with others.

Sure, you could publish your app as a .tar.gz of the filesystem root and then users could extract it and have a script to bind mount everything and chroot into it. You could then set up a systemd service(s) for that script and set up a slice. But then you're just reinventing Docker from scratch for absolutely no reason. You could also use systemd-nspawn, but then you're losing out on a bunch of really useful features that the devs omitted because of philosophical reasons (as usual with systemd, they think they know better than you).

I'm not really telling people to not use Docker. I'm telling them how common cgroups are -- every Linux system is already using them.

Cool stuff with the "slice units." I use systemd to keep apps running but didn't know all this. And yes I understand the basics of what Docker containers are. It just seems logical to me that it would be a lot more taxing on the system running that overhead. Like is it exponentially harder to fit the same amount of apps on a droplet if they're all containerized? Or is it still easy to run 4-5 modest containerized apps on a $5 droplet?