the 's form should be used to form the possessive of a singular noun. However "jeans" is treated as plural (just as "pants" is) and so the possessive would be jeans'. But when one indicates "the pocket of a pair of jeans" one usually uses "jeans" adjectivally, so it modifies "pocket" giving "the jeans pocket" and not "the jeans' pocket". Indeed the latter form is so unusual that I can't recall having seen it. Similarly, in "the shirt pocket" shirt is being treated as an addictive, not a noun to be put in the possessive form. However, when referring to a particular shirt, a possessive form might be used:
My blue plaid shirt's pocket.
Whether to use a possessive form, or to use a noun as an adjective, is just a question of usage and custom -- either is grammatically correct, but in a given case one may be natural and the other not. It might also vary in different varieties of English for some words.