Good question. IMO it was Luke.
Firstly, it may be that the door automatically opens to everyone but why would Maz keep such an artifact behind an unlocked door in the middle of a crowded bar? Furthermore, we know it's probably a bit of a dangerous bar since Han says "don't stare" (also it blows up after 15 minutes). Dangerous bars naturally tend to have locked doors to keep the thieves out. Therefore on the balance of probabilities it was a locked door that somehow magically unlocked. Let's now consider some points:
A) In the bar, Rey hears a voice - it's probably herself as a child crying out as she is abandoned on Jakku. This is why she goes down the stairs.
B) After the vision, Maz says "That lightsaber was Luke's. And his father's before him. And now, it calls to you....I am no Jedi, but I know the Force."
C) So at this point in the movie there are powerful Force events taking place:
1) The lightsaber "calling" to Rey;
2) She has a major hallucination that freaks her the F out.
Therefore, probably the door opening is part of all this crazy Force stuff that's going on.
All of this would make it a great candidate for the meaning of the title The Force Awakens EXCEPT Snoke says "There has been an awakening. Have you felt it?" 14 mins before she has the vision.
So if Rey's vision is not the awakening, then what is the title actually referring to? There must be something Force-related that we cannot see. Maybe it guides some of the characters. Maybe it guides Rey to Luke. Maybe this is why the door opens.
In the first 3 movies Palpatine has this huge scheme going on in the background which eventually sees him become the Emperor with a huge army of stormtroopers. During this scheme, a child (Anakin) randomly flies inside a droid control ship and accidentally blows it up. Due to the sheer number of coincidences involved, it seems very likely that he was being guided by the Force. Maybe he was being guided by Palpatine?
Perhaps, Luke has a scheme of his own going on in the background of this trilogy? I feel it's important to ask why does Luke touch R2 in Rey's vision? Is this just a friendly stroke in front of the campfire that has nothing to do with Rey? IMO no. R2 only wakes up when Rey turns up. It seems likely that Luke programmed R2 to do this and possibly that touch is part of this process - i.e. it's all part of Luke playing the long game over many years - he touched R2 probably when Rey was still a child (the vision is probably in chronological order e.g. the first scene is on Bespin and the touch occurs before we see Rey as a child, with Kylo in the snow in the future in the last scene in the vision). This grand long-term plan of Luke's is why the door opened in my opinion. He used the Force. He wants Rey to reach the island. Kylo says Rey herself has been dreaming of the island. Luke's plan involves Rey partly because she is related to Luke. In one of the trailers he says "The Force is strong in my family. My father has it. I have it. My sister has it. You have that power too."