Docker only helps with portability between Linux distributions, not operating systems.

You can't run it on Windows, Mac or the various BSDs without a VM, at which point you're running "the same OS your server uses".

Yes and no.

I use OS X for my main machine, and can happily deploy something to a server running 'not OS X' and be confident that all libraries and dependencies are exactly the same in production as what I was using in development.

Sure Docker is running through a stripped down VM on the Mac, and so technically it's "the same OS my server uses", except it's abstracted away so that for all intents and purposes I'm using OS X for development (and email, browsing, and other things) and deploying hassle free to Linux.

That's not true; Docker runs great on OSX natively. You can't run the containers there, but the client itself is fine and largely abstracts away the whole "where am I building" issue.

Minor nitpick - Docker runs great on OS X, but it doesn't run natively, it runs on a minimal Linux VM - Boot2Docker.

I agree that the reality of the situation is that it's abstracted away enough that the distinction isn't really all that meaningful.

No, the docker program itself does indeed run natively on osx. You do not need to use boot2docker or any other VM.

You can't run any containers on osx, but docker itself runs fine as a client binary to docker running on a linux server. I use this configuration daily using a native osx docker binary on my workstation speaking TLS to a docker service running on a CoreOS server.