On what technologies is the Docker containerization tool built?

Docker is built on several key Linux and open-source technologies:

• Linux Containers (LXC) — originally Docker used LXC for process isolation but later transitioned to its own runtime.
• Namespaces — a Linux kernel mechanism that isolates resources (processes, network, filesystem, etc.) for containers.
• Control Groups (cgroups) — allow limiting and controlling resource usage (CPU, memory, disk) for containers.
• Union Filesystems (UnionFS) — such as OverlayFS, which provide a layered filesystem for containers, enabling efficient use of images.
• libcontainer / runc — tools for running containers that interact with the Linux kernel.

Thus, Docker leverages Linux kernel features for isolation and resource management, along with its own tools for easy creation, distribution, and running of containers.