A quarantine that tries to prevent people getting food is a quick way to get riots. No government would be stupid like that.

If it gets bad enough, it's quite likely that there'll be martial law, and possibly food rationing enforced.

Indeed. If a whole city is under quarantine then the supply of food has to be managed by the authorities.

Show me a person who values their minimum wage so much that they will risk making pizza deliveries (or even bake the pizza) in a quarantined city, please. Typically, the government does it in these cases, and often it's not enough or not what you need or prefer, so it's advisable to prepare.