Docker is also, pardon my French, more "web 2.0"-ish software than all other 3:
* The Docker website is clean, modern, and attracts. It clearly tells me how to get started. They even invested time/money in a proper logo. Now look at the other websites. They look old, archaic, too formal, and looking at them don't give me the feeling that I should try out their software. The UML website gives off the feeling that it is unmaintained. I realize this isn't a purely technical reason, but presentation is important.
* Docker feels like it has more community around it. It uses Github and you can see that it's very active there. Docker also has an easy to use image repository. Apart from being extremely useful to pull in an image with a single command, it gives off the good feeling that there's an active community around it and that it's actively developed.