Rename WPFPM_FLAG to avoid environment injection

[emcniece]

A user has pointed out that the WPFPM_FLAG variable is set and gets injected into the wp-config.php file at runtime.

The purpose of this variable is to provide an identifier so that select environment variables can be iterated and injected into wp-config.php. Since the value of WPFPM_FLAG defaults to WPFPM_ (which it itself starts with) the iterator includes this variable during injection.

WPFPM_FLAG should be renamed so that it does not get injected as it is misleading.

Modify:

• https://github.com/emcniece/docker-wordpress/blob/master/Dockerfile
• https://github.com/emcniece/docker-wordpress/blob/master/docker-entrypoint2.sh#L12