Then add that Ruby on Rails has a history of breaking when trying to update to keep ahead of security vulnerabilities, and we may expect to see some big security issues when something is found that's exploitable and updating breaks things, and everyone is sitting around waiting for the Mastodon people and/or the Docker image makers to update things properly.
Mastodon would be improved by spending more energy making fewer assumptions about the underlying OS and being less dependent on specific versions of underlying Ruby components.