[timdorr]

From their about page: https://www.docker.com/company/aboutus/

  Docker, Inc. offers Docker-related products and services and is creating a network 
  of certified professional support, training, and services providers. We are 
  committed to keeping Docker open source under the Apache 2.0 license.

They used to be dotCloud, and presumably have some legacy clients still paying them money to keep the lights on.

[mattzito]

They sold their dotCloud business so they could focus on core Docker.