Docker excels in image building especially now with multi-image Dockefile support. It seems all those alternatives to the Docker just gave up on providing anything on their own. The documentation typically starts "lets pull a docker image".

On the other hand the container runtime is straightforward. I recently discovered that one can run a docker image with a bash script and the unshare command and get a very tight security setup. That explains proliferation of various alternatives to Docker to run its images.