As many of you will know, the world of ops and infrastructure is a matter of read the manual, try, get it wrong, repeat cycle.  That's great if the thing you are working with is a physical piece of hardware with clear instructions that do work.  Software Emulated hardware can have some interesting side effects as I soon found out when using Docker containers to host MySQL and Glassfish.

MySql

The official download from hub.docker does have very clear instructions, but me being me, I didn't read the writing on the tin, especially the section "Using a custom MySQL configuration".  Oh, and more importantly this section -

Initializing a fresh instance

When a container is started for the first time, a new database with the specified name will be created and initialized with the provided configuration variables. Furthermore, it will execute files with extensions .sh, .sql and .sql.gz that are found in /docker-entrypoint-initdb.d. Files will be executed in alphabetical order. You can easily populate your mysql services by mounting a SQL dump into that directory and provide custom images with contributed data. SQL files will be imported by default to the database specified by the MYSQL_DATABASE variable.