@David_St Copy the H2 file (/application.db/metabase.db.mv.db) out and do it outside - it’s a one-time procedure.
This might help you: Run the Migration from H2 to MySQL from inside Docker?
Once you have migrated, then setup a new container that will connect to the new application database:
https://metabase.com/docs/latest/operations-guide/running-metabase-on-docker.html