Once upon a time—specifically, until a few months ago—Docker seemed to be the only serious open source container-virtualization solution in town. But now alternatives are cropping up all over the place, with the announcement this week of Rocket from CoreOS only the latest. Have the Containerization Wars begun?

CoreOS is building a new, lightweight Linux distribution, forked from Google's (GOOG) Chrome OS, that is designed for large-scale cluster deployments. On Dec. 1, the project announced that its OS will also include a homegrown container virtualization tool called Rocket, which it said will prioritize "composability, security and speed."

The CoreOS developers are not tiptoeing around the fact that Docker already exists as a mature, basically enterprise-ready containerization platform. Instead, they're taking Docker to task for what they see as vital shortcomings. In particular, they say, Docker has failed to live up to its promise of being an easily configurable platform with readily interchangeable pieces. Instead, it has become "monolithic," making it difficult for developers and admins to pick and choose which parts of Docker they want to run without committing to the entire Docker ecosystem, which is now quite expansive.

If CoreOS developers were the only group questioning whether Docker is really making the most of containerized virtualization, there would not be much to see here. But a chorus of Docker critics is now growing, with companies proposing their own, competing solutions.

Besides CoreOS's Rocket, Canonical has launched LXD for Ubuntu. Spoonium is developing a containerization platform that basically does the same thing as Docker, but in a more platform-agnostic way (for the record, Docker is also moving toward cross-platform interoperability with promises of Microsoft Windows support coming in the future). Flockport has announced a containerization solution that, like Docker, is based on Linux Containers (LXC), but which focuses more on virtualization workloads than app delivery. And then there's LXC itself, which continues to chug along, building the code that Docker and many of these other endeavors depend on, toward ends that are not guaranteed to serve downstream projects.

In light of all of these initiatives, it seems a safe bet that, despite Docker's immense growth over the last year, the company will hardly hold a monopoly on open source container virtualization. This is a niche poised to see as much rivalry as, say, the NoSQL storage world, or the big data ecosystem.