> Docker is NOT about local dev, it's about deployment.

I mean... if Vagrant can be about local development, I don't see why Docker can't be about local development too.

It can be anything you want it to be, especially inside the confines of your own setup.

Do you want to quickly debug some software that relies on an older version of Postgres 9 but you don't want to clutter your host machine? Just run the following command to fetch and start an instance. With --rm flag it's going to be removed as soon as you terminate it:

docker run --rm -p 5432:5432 postgres:9

It can be.

But that was never the primary purpose of Docker. Like I can use Mac Minis as door stops, etc.

Docker runs best on top of Linux. As in actually run a Linux host , everything else in my experience is a hack. As long as you have internet access, spinning up an ec2 instance to test stuff shouldn't be too hard.