No, prisoners convicted of prohibition related offences were not released after its repeal.

Generally, crimes committed before a law is repealed are not automatically released once it is repealed. They must be offered amnesty, or pardoned by a governor (State Laws) or the president (Federal Laws) except for under special circumstances. Amnesty is similar to a pardon, but is reserved for a large group rather than an individual. The power to grant amnesty belongs solely to the president.

This seems to be the case for alcohol related crimes committed during prohibition. For a criminal convicted of selling or distributing alcohol, or of related offences during prohibition, the ratification of the 21st amendment did not mean an end to their sentence. Most did serve their time for the now repealed laws.

As far as sources go, the Los Angeles Times had a headline of "Old Federal Dry Law Violators to Stay in Jail" published the day after the ratification of the 21st amendment, December 6, 1933. Unfortunately, the archived article is behind a paywall. You can download the PDF here for $3.95 if you wish to read it in its entirety.

Quick reminder: If you yourself don't have an answer to give to address OP's question, please refrain from commenting.

And if you do have an answer, please try to make sure it's up to the standards of the subreddit. Answers need to be both informative and comprehensive, and not single sentence responses with nothing backing them up.

As a follow up, what about cases that were pending? Did the court schedule (docket?) become a lot less full?

I'm not as versed in law as others are here, but I have done practice bill drafting for school, and so while I may be misinterpreting the decision, in 1798 (much before Prohibition) there was a supreme court case (Calder v. Bull) that defined four aspects of "Ex Post Facto" law. Among the four was the definition that only laws that increased sentences were ex post facto, and therefore unconstitutional. However, commutation of sentences were not specifically deemed unconstitutional, as they were not considered ex post facto. I'm quite sure I'm misinterpreting, and would be quite glad for someone's prompt correction, but to my knowledge, it was not unconstitutional to release prisoners. Which doesn't answer your question directly, but rather disproves some of the other responses' reasons for why the release couldn't have happened.

Sources: Calder v. Bull, 3 U.S. 386, 1 L. Ed. 648, 1 L. Ed. 2d 648 (1798).

Full Text: http://caselaw.lp.findlaw.com/scripts/getcase.pl?navby=CASE&court=US&vol=3&page=386