A mere nine months after its last round of funding and just 18 months after its launch, Docker Inc. is announcing today that it's received a cool $40 million to support its booming expansion. Not a bad show for a company that just released its first commercial offering this summer.

The Sequoia Capital-led series C funding valued Docker Inc. at around $400 million, GigaOm reported when it sussed out the funding round in August. Docker Inc. had just raised a $15 million Series B round in January.

Containers — which house applications along with dependencies such as binaries and libraries — have been around in Linux since about 2000, but Docker made them simpler to use. As a result, Docker containers are suddenly all the rage for deploying distributed applications in the cloud and in data centers; witness their sudden appearance in Nuage Networks demos. Downloads of the platform have leapt to 21 million today from 3 million at DockerCon in June, according to the company.

There's even talk that containers could replace virtual machines. Not surprisingly, VMware is more of the opinion that the technologies should work together.

The key difference between containers and VMs is that the hypervisor abstracts an entire device, operating system and all. In other words, VMs emulate virtual hardware, while containers share an operating system. That can lead to some serious efficiency gains.

Early Docker container users include Cambridge Healthcare, eBay, and Spotify, with some companies reporting increases in data center efficiency by 10x or more, according to Docker. Results like that have been enough for the young company’s 1.0 version to earn second looks from even traditionally risk-averse sectors like banking and government.