XDebug causes fatal crash on official php:5.6-apache image

[graney]

If debugging in code that accesses a declared static property XDebug crashes with a Uncaught Error: Access to undeclared static property: on the line where the declared static property is accessed.

This has to do with the base php:5.6-apache docker image.
This issue discussed in more detail here docker-library/php#133

[emrzv]

Well, it's better late than never. No one has checked the issue tracker since it's not a service we provide.
Fixed by applying the fix from the linked thread, thanks!