These docker images are built by the portability CI workflows for Linux.

However, refactoring of the workflow had broken the upload to docker.pkg.github.com. (Although the login to docker.pkg.github.com succeeds, the push fails with no basic auth credentials - see https://github.com/mkoeppe/sage/runs/1414148202?check_suite_focus=true)

CC: @kliem @dimpase @tobiasdiez

Component: porting

Author: Matthias Koeppe

Branch/Commit: 21afe51

Reviewer: Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/30933