Which FIFA law prohibits a player from carrying the soccer ball around in his shirt tail (like a basket)?
That would be regarded as either handling the ball or an obstruction.
This isn’t specifically mentioned in the Laws of the Game as published by FIFA.
It’s debatable whether it would constitute handling the ball, and the obstruction rule (both part of Law 12) is unclear with respect to the OP’s question. However, in the section called Additional Instructions for Referees, Assistant Referees and Fourth Officials, we find this:
So at the discretion of the referee, a player using his shirt tail like a basket would break this general amendment to Law 12.
Also, the list of cautionable offences begins “[A player is cautioned and shown the yellow card if he] is guilty of unsporting behaviour”. Interpretation of that is also the discretion of the referee.
My WAG is that if it was legal the Dutch would have done it, especially when Johann Cruyff was playing. Cruyff had an insatiable appetite for the unorthodox, including passing the ball to a team mate when taking a penalty kick and even arguing with his heart specialist about the best way to perform the bypass surgery he needed.