How could Jesus get through locked doors? Since Jesus tomb was empty, he obviously arose in his material body, and throughout the Bible references to shutting the door at night, is the same as us locking the door at night since it is obvious in Acts 12:13-16 that Peter was unable to open the door until someone inside opened it for him.
Acts 12:13-16, KJV

13  And as Peter knocked at the door of the gate, a damsel came to hearken, named Rhoda.
14  And when she knew Peter's voice, she opened not the gate for gladness, but ran in, and told how Peter stood before the gate.
15  And they said unto her, Thou art mad. But she constantly affirmed that it was even so. Then said they, It is his angel.
16  But Peter continued knocking: and when they had opened the door, and saw him, they were astonished.

Yet in John 20 He appears inside a locked door in his physical body since Thomas was physically able to touch him.
John 20:26-27, KJV

26  And after eight days again his disciples were within, and Thomas with them: then came Jesus, the doors being shut, and stood in the midst, and said, Peace be unto you.
27  Then saith he to Thomas, Reach hither thy finger, and behold my hands; and reach hither thy hand, and thrust it into my side: and be not faithless, but believing.
Great question! I'm sure it was a miracle because John would not have bothered to mention the locked doors otherwise.


  On the evening of that first day of the week, when the disciples were together, with the doors locked for fear of the Jewish leaders, Jesus came and stood among them and said, ‘Peace be with you!’


John 20:19

So how did it happen? Jesus clearly had a physical body:


  And while they still did not believe it because of joy and amazement, he asked them, ‘Do you have anything here to eat?’ They gave him a piece of broiled fish, and he took it and ate it in their presence.


Luke 24:41-43

So how did Jesus just appear  in a different location? Well, this is not the only time in Scripture this has happened:


  When they came up out of the water, the Spirit of the Lord suddenly took Philip away, and the eunuch did not see him again, but went on his way rejoicing. Philip, however, appeared at Azotus and travelled about, preaching the gospel in all the towns until he reached Caesarea.


Acts 8:39-40

Here the Spirit of the Lord performed a miracle.

Furthermore, we also read that Jesus also could suddenly disappear (Luke 24:31):


  Then their eyes were opened and they recognised him, and he disappeared from their sight.


Whether Jesus Himself in His resurrected body could perform miracles by Himself (which I suspect) or whether The Holy Spirit did it for Him, the conclusion must be - it was a miracle.