added builder script

mariadb/README.md

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+# Mariadb
+
+Dockerhub: https://hub.docker.com/_/mariadb
+
+It will be quite uncommon to just use this compose file, it's likely you are extending this from another project.
+That's the main context of this container.
+
+## Sample 
+
+### Run multiple databases 
+
+initdb/init.sql
+
+```sql
+
+-- Create BookStack database and user
+CREATE DATABASE IF NOT EXISTS bookstack_db;
+CREATE USER IF NOT EXISTS 'bookstack_user'@'%' IDENTIFIED BY 'bookstack_password';
+GRANT ALL PRIVILEGES ON bookstack_db.* TO 'bookstack_user'@'%';
+
+-- Create WordPress database and user
+CREATE DATABASE IF NOT EXISTS wordpress_db;
+CREATE USER IF NOT EXISTS 'wordpress_user'@'%' IDENTIFIED BY 'wordpress_password';
+GRANT ALL PRIVILEGES ON wordpress_db.* TO 'wordpress_user'@'%';
+
+FLUSH PRIVILEGES;
+```