Originally created for Alpine Linux, the Alpine Package Keeper (apk) is today utilised by many other distributions as well.
What is Alpine Linux APK?
Alpine Linux provides a more complete package repository than BusyBox-based images and access to an image under 5 MB in size. Designed for utility as well as production uses, the image is built on musl libc and BusyBox. Read more about Alpine Linux here since its motto fits exactly at home with Docker images.