I'm running it on a public server. There docker separates: serving a website with nginx, running some node processes separately (proxied by nginx), running postfix and running git. Note that git is also separate from ssh of the host system.

It has helped us in situations where we wanted to try new configurations of nginx and email without fiddling with the production processes. Reverting changes was also handy sometimes.

It also helped us to be more flexible with versions. We're more confident about running a very recent version of nginx, which allows us to make use of the new features instead of waiting for debian to ship packages. Also less hassle with with dependencies, although the different docker instances need to be upgraded like any other VM.

That said, when we set things up we were missing some features (connecting docker instances to eachother). We had to fiddle to get things the way we wanted and we're still missing some features that we have made workarounds for. I wouldn't recommend it running it in production. However for us it's worth the effort especially in the long run when Docker can handle all of our use-cases.