BuildKit is Docker's new and improved builder backend. In addition to being faster and more efficient, it supports many new features, among which is the ability to provide a persistent cache, which outlives builds, to compilers and package managers. This is very useful to speed up consecutive builds, and is used by some container images of OpenAssistant's stack.

The BuildKit backend is used by default by Compose V2 (see above). But if you want to build an image with docker build instead of docker compose build, you might need to enable BuildKit.

To do so, just add DOCKER_BUILDKIT=1 to your environment.

For instance:

You could also, more conveniently, enable BuildKit by default, or use Docker Buildx.