docs inaccuracy for MARIADB_DISABLE_UPGRADE_BACKUP

MariaDB docs (on Docker Hub here and at mariadb.com here) for MARIADB_DISABLE_UPGRADE_BACKUP are inaccurate (inconsistent with the current code):

Docs:

This backup process can be disabled with by setting MARIADB_DISABLE_UPGRADE_BACKUP to a non-empty value."

(e.g. https://hub.docker.com/_/mariadb)

vs. code:

{
	if [ -n "$MARIADB_DISABLE_UPGRADE_BACKUP" ] \
		&& [ "$MARIADB_DISABLE_UPGRADE_BACKUP" = 1 ]; then
		mysql_note "MariaDB upgrade backup disabled due to \$MARIADB_DISABLE_UPGRADE_BACKUP=1 setting"
		return

[ https://github.com/MariaDB/mariadb-docker/blob/master/docker-entrypoint.sh#L369 ]

Note also this issue, which affects multiple places (see list):

= and == are for string comparisons
-eq is for numeric comparisons

[ https://stackoverflow.com/a/20449556/9962007 ]

Start with docker-entrypoint.sh around line 369 and compare its MARIADB_DISABLE_UPGRADE_BACKUP condition with the wording on the Docker Hub and MariaDB documentation pages. Update the documentation so it matches the current condition and verify the related shell comparison guidance where applicable.