A320 Captain reported a 'dirty socks' odor during arrival taxi to gate. Maintenance MEL'ed the APU and returned the aircraft to service.

Narrative

I was flying an A320 from ZZZZ to ZZZ. I have not experienced something like this in this aircraft type. Power did not change throughout the event as the APU was the bleed source. It did rain for a few minutes while we taxied in to the gate. After deplaning an odor of dirty socks was noticed in Cabin. Crew dispersed throughout aircraft to determine source. No physical item was found so we thought it must be air supply. Captain and I performed fume event checklist. Once we turned off APU bleed the odor stopped. Cabin was also clear of smell. We tested the bleed again and odor immediately returned. We called Operations and requested local Maintenance to come to aircraft and delayed boarding. Local Maintenance agreed it smelt like an APU issue but said it was okay. We then wanted Maintenance Control involvement but had many dropped calls throughout process. Maintenance Control wanted us to start engines at the gate and test engine bleed through packs to see if the odor returned. We preformed test with Chief Pilot approval. I was not and have never been trained on performing maintenance tests. I have not had training on certifying air cabin quality for human use yet I was asked to do so. The odor was minimal to none during the test. I asked the Flight Attendants to leave the aircraft. I asked that the Spanish fluent FA stay in case we needed translation operating 2 engines at gate. Maintenance Control and local Maintenance placed the APU under MEL. We then had the issue of the cabin overheating so we requested ground air. The crew agreed it was safe to continue and no one felt sick or disoriented. This took some time to arrive. We began to cool the cabin as quickly as possible. We then boarded and pushed back; there was a faint lingering odor that was intermittent. No one else smelt it so I did not think anything of it as we were operating the aircraft at an international airport. It stopped about 10 min into engines supplying bleed. Flight was completed after lengthy delay. We did not don the masks as we were on ground and had cabin door open. There was also concern if we went on mask then the limited communication capability we had would be lost.