I am not satisfied with Docker.
Untrusted users should be able to run their own container instances. Running a basic container instance means forking a process, putting it in the right kernel namespaces, and setting up mounts and virtual network interfaces, and those can all be done without privileges. Unfortunately, in Docker, access to the Docker daemon is tantamount to being root. Fixing this would improve both convenience and security.
In fact, a global system daemon should not be needed. Either users should be able to run their own daemons or container management should avoid having a daemon at all, by storing container state in a shared database.
Docker container builds are too slow. Installing a container image requires piping massive amounts of image data to the daemon over a socket, which is ridiculous. This could be avoided by passing a file descriptor to the container manager ... or you might even get rid of container image management and start containers by pointing directly to the files comprising the image.
Docker container instances start too slowly. It shouldn't take seconds to start or stop a simple small container. The system calls required to start a simple container run in milliseconds.
No doubt these observations are old news, so I assume there are better container implementations out there. Which one do I want?