Add support for DDEV

[hussainweb]

This fixes #15 but with a few workarounds. The change here needs to repeat a few steps from the base image because we are recreating the /mysqlbase directory contents. This also increases the image size but there is no good way around this. More details in #15.

The create_init_db.sh script is copied from DDEV's create_base_db.sh with only these changes inserted into the middle of the file (just before mariabackup is run:

set +u
source /usr/local/bin/docker-entrypoint.sh
docker_process_init_files /docker-entrypoint-initdb.d/*
set -u

It needs to call set +u to disable unset variable checks. MariaDB's docker-entrypoint.sh does not reliably check for this in the docker_process_sql function resulting in an error because of $1.

/usr/local/bin/docker-entrypoint.sh: line 197: $1: unbound variable

Rather than fixing the error, it's simpler to just disable the check. This may not be a long-term fix but works now and hopefully a better solution exists which can be used upstream in DDEV.