Are there any simple electronic door locks available?
You don't necessarily need an electronic lock - you just need a lock that is always locked from the outside and yet allows exit from the inside.  Something commercial in nature is probably in order.  You probably won't find this at a home supply store, however.  You likely will need to consult a locksmith.

CORRECTION

Link to Home Depot collection of storeroom locks.

You can find them at Home Depot.

Thanks to RSMoser for the proper name!