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I haven't looked much into any of these yet but I'd like to once my exams are over. Which one has proven the most useful to you? Or are you perhaps like me and don't use these at all?
Docker without a doubt.
Then once you've had a play, learned to love it, deployed a few swarms, and you decide you're confident and want to hate yourself, have a play with kubernetes. . . As for coreos, can have a go if you want, but most of the time you'll never see it, it will just be preinstalled for you on an instance in the cloud somewhere
worth trialling gcp too. you can directly launch a docker app, or a vm instance. they have the usual debian/ubuntu/centos but also a docker ready version of Chromium OS so you can mess with docker some more.
not yet tried kubernetes or swarm, but its on my todo list
For me, Docker.
i never used Vagrant or Coreos