MySQL: update readme; complete initializing a fresh instance paragraph (#2627)

Co-authored-by: Tianon Gravi <admwiggin@gmail.com>

Author: gouglhupf

mysql/content.md

@@ -134,7 +134,9 @@ Currently, this is only supported for `MYSQL_ROOT_PASSWORD`, `MYSQL_ROOT_HOST`,
 
 # 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 `%%IMAGE%%` services by [mounting a SQL dump into that directory](https://docs.docker.com/storage/bind-mounts/) and provide [custom images](https://docs.docker.com/reference/dockerfile/) with contributed data. SQL files will be imported by default to the database specified by the `MYSQL_DATABASE` variable.
+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`, `.sql.gz`, `.sql.bz2`, `.sql.xz`, and `.sql.zst` that are found in `/docker-entrypoint-initdb.d`. Files will be executed in alphabetical order. When parsing `.sh` files without the execute bit set, they are `source`d rather than executed.
+
+You can easily populate your `%%IMAGE%%` services by [mounting a SQL dump into that directory](https://docs.docker.com/storage/bind-mounts/) and provide [custom images](https://docs.docker.com/reference/dockerfile/) with contributed data. SQL files will be imported by default to the database specified by the `MYSQL_DATABASE` variable.
 
 # Caveats