Docker is practically dead, no wonder if disappeared :-)
Many things are hellishly complicated in Linux, due to politics and technical difficulties. Case in point: when I’ve started to work on NFSv4 ACLs, support in Linux was “worked on”, there was a prototype. It was 12 years ago. In FreeBSD, full supper for NFSv4 ACLs, from file systems to userspace tools, shipped decade ago. In Linux it’s still not there.