Fix the broken import of .sql and .sql.gz seed files during database init

The code responsible for processing .sql and .sql.gz files mounted to /docker-entrypoint-initdb.d does not work.

This PR fixes that process with these steps during the DB init phase of run.sh

Wait until $MYSQL_DATABASE and $MYSQL_USER have been created and granted privileges.
Check the contents of /docker-entrypoint-initdb.d to see if there are any files worth processing.
Launch the mysqld server temporarily to process the seed files
- .sql files: Directly push the contents into $MYSQL_DATABASE via the mysql client CLI
- .sql.gz files: Gunzip and pipe the direct output into $MYSQL_DATABASE via the mysql client CLI
Shut down the temporary server and tidy up

Currently open issues that will be resolved by this PR:

yobasystems / alpine-mariadb