WP images ship with a mysql client that is assembled from MariaDB sources and doesn't have support for caching_sha2_password.

$ mysql -V
mysql  Ver 15.1 Distrib 10.11.8-MariaDB, for Linux (x86_64) using readline 5.1

This is because MariaDB Server does not currently support caching_sha2_password as documented here.

When using mysql 8.0+ as the server, and running wp db cli for example, we get this error:

ERROR 1045 (28000): Plugin caching_sha2_password could not be loaded: Error loading shared library /usr/lib/mariadb/plugin/caching_sha2_password.so: No such file or directory

This is because mysql 8.0+ uses caching_sha2_password as it's default authentication plugin as documented here: In MySQL 8.0, caching_sha2_password is the default authentication plugin rather than mysql_native_password.

I found PR #902 regarding the issue. Installing that package likely fixes the issue because it provides the caching_sha256_password plugin, which is compatible with MySQL's caching_sha2_password as documented here.

@tianon I am unsure if that would actually be a robust fix, but here's the issue explained more in detail.