The racist door is a figurative/metaphorical door that is opened when somebody makes a racist comment. This either allows for further racist comments or it allows for the retort: "You are a racist" or words to that effect, thereby closing the racist door.
Coined on Mock The Week, by Frankie Boyle, when someone made a racist comment against Scotland (his homeland) and he replied: "Thanks for opening the racist door for me."
"Hello children, shall we see what's behind the racist door today? Oh, it's ching chong chinaman."
<hears racist comment> "Does anybody hear a creaking door?"
- Frankie Boyle