According to the Royal Shakespeare Company website, "goose" was a slang word for prostitute in Shakespeare's time. Therefore, it is likely, as Shakespeare loved double entendres, that the porter is punning on that meaning of the word "goose" in this line.
A goose was an iron a tailor would use, so the overt or obvious meaning of the line is to invite the tailor in to roast or heat up his iron in the fire. However, the sexual meaning would be the following: come on in tailor, here you can warm up your prostitute, with roasting also having a sexual meaning of warming her through close bodily contact.
In Romeo and Juliet, Romeo uses the word goose in Act II, scene 3 to mean prostitute—so Shakespeare was aware of the slang.