Immutable Linux distributions
Most of these focus on running containers. What I would like is a distribution that focuses on the reliability of upgrades, both configuration and data. To give a concrete example, I have a Debian server running Apache (with auto-certificate renewal), PostgreSQL, and Postfix. When it's time to upgrade to the next release, it's basically the "it should work, but who really knows, depends on your specific setup you may have to tweak a few things" kind of guarantee. What I want is a rigorous guarantee like we have for relational database schema/data migration.