The root docker containers for ubuntu are based on the official ubuntu docker containers (FROM ubuntu:20.04) and these are “minimized” for non-login use, as explained in the ubuntu wiki under “Minimal”.
The root docker containers are obviously for login use, and thus they should be “unminimized” during the container build, e.g. by adding a “yes | unminimize” invocation in the dockerfile. Some consequences are missing man pages and inconsistent locale settings.
At the moment we fix this post-mortem in our local containers derived from the root-ubuntu docker containers, but this fix should be done at the source so that the root-ubuntu docker containers are ready for use.