> Also these VM images usually contain tons of undocumented "state"
Although I agree this approach is absolutely flawed, and anyone doing so needs to re-evaluate their workflow, I feel this comment is more an argument of doing things properly, rather than a positive of Docker. Everything you mentioned above can be easily achieved by using Vagrant and a provisioner. But as discussed in other comments, the Docker ecosystem helps you get started quicker.
Although I agree this approach is absolutely flawed, and anyone doing so needs to re-evaluate their workflow, I feel this comment is more an argument of doing things properly, rather than a positive of Docker. Everything you mentioned above can be easily achieved by using Vagrant and a provisioner. But as discussed in other comments, the Docker ecosystem helps you get started quicker.