How do you share files between the docker instance and the host? How do you run docker on your mac? Networking is very different for docker instances as well.

On a Mac you can use Vagrant to bring up a host that runs docker, but i'm not sure how you would be able to just run docker on your mac.

That being said docker looks pretty awesome for certain sets of problems.

The nice thing about Vagrant is that it is more platform independent, so even if you are doing your dev work on a mac you can still use the same dev-box as anyone else. Docker, on the other hand, is linux only (and requires a certain kernel version) to work well. Not to say you can't use Docker, and they seem to work well together, but they are kind of solving different problems.

can docker have different operating systems with different kernels (win, freebsd, debian lin, ubuntu) running on the same machine? or same o/s with different kernels? different modules?

the old vps vs. container discussion.

It is possible to run a VM inside Docker and still retain some advantages of containers, but not all of them since there is a "VM" in the sentence :)