Am just starting to explore Docker.
Docker is a pretty modern piece of tech, that has very little wrong with it, and seems to work exactly as designed. So you are kind of shoe horning the reference...
I'm a pretty smart person; I've poured over the Docker documentation, run through the interactive tutorial twice now, and I still don't have a great sense of 1) what it really is for, 2) how to really use it, or 3) how to handle slightly-non trivial use cases.
For #3, reading through a few examples online of how to get MySQL or Redis up and running in a container... honestly makes my head hurt. And those represent just one or two parts of the system I'm thinking could some day run on Docker - I don't have the time or patience right now to figure out how to get Nginx, Node.js, Redis, MySQL, RabbitMQ, and a few other things here or there - stitched together into a dockerfile.
If I had to make a guess at what the "bad parts" of Docker are, it's that it's complex and not all that understandable - yet. Maybe there aren't that many things that are technically wrong with it, but at this point, it's pretty painful to wrap one's mind around (IMO), and at least I personally think thats a "bad" part.
Docker feels like one of those things that is going to be indispensable and incredibly useful in a year or two. I'm certainly keeping my eye on it, but I'm staying away from the diving board for now.
That goes for anyone else reading this. I love answering questions and helping people. It's like pure bliss, so don't worry about being a bother.
I'd be happy to take whatever I learn and apply to my app and contribute it back to the community, maybe as a quick tutorial up on GitHub, or something along those lines.