I don't mean to be overly dismissive or anything, but what does Docker make that customers will pay for? This is an awful lot of money to throw at a company that doesn't have a business plan, let alone technology that can't be readily replicated.
My tiny company pays them for private repositories as part of our deployment process. (Push to Github, Docker Hub makes automated build, built Docker image gets sent to AWS ElasticBeanstalk). It's worth the $7 a month for the convenience of not doing the builds and hosting the images ourselves.
Not saying it's going to turn into a business model that will justify that valuation, just that it does at minimum provide a value I'm willing to pay a small amount for.
Betting $100M+ needs real return. xD
Docker seems to be much closer to Atlassian. Which could well be their biggest competitor one day.