MySQL 8.0 changes default auth plugin from `mysql_native_password` to `caching_sha2_password`

[huwd]

#550 documents a workaround by altering the root users config to set it back to mysql_native_password which lets things build again.

We should try integrate a solution into govuk-docker itself, either by placing this command into the makefile.
Or seeing if we can meet the config requirements for mysql_native_password.

The Plugin docs are here