At PyCon, Solomon Hykes showed off docker to the public for the first time and talked about Docker as the solution to hackers asking for that "low-level piece to do that magical thing" they do at dotCloud.
@shlomoswidler IMHO docker is to paas what chef was to Iaas 4 years ago— botchagalupe (@botchagalupe) March 17, 2013
From the project's homepage:
Docker complements LXC with a high-level API which operates at the process level. It runs unix processes with strong guarantees of isolation and repeatability across servers.
Docker is a great building block for automating distributed systems: large-scale web deployments, database clusters, continuous deployment systems, private PaaS, service-oriented architectures, etc.