There are, in fact, quite a number of scenes in the movie which involve door frames. Brody typing up his report of Chrissie Watkins's death is framed by an open doorway as are Quint and his mate when they leave the Town Hall meeting. Brody and Ellen storm through an open door when arguing about Michael sitting in the boat, and she later leans against the open kitchen door as she watches her husband and son share a moment. Most famously, Brody backs slowly through the open door of the Orca's cabin before uttering his line about a bigger boat. Quint himself slides to his death through that same open door and Brody escapes a similar fate by bracing himself against the jambs. None of these images have the symbolic resonance of, say, the door frame in The Searchers or the bedroom door in The Exorcist, and - given the unavoidable fact that life involves a great deal of opening and closing of doors- there may be nothing more to this architectural pattern in Spielberg's movie than simple happenstance.
If we're really intent on looking for meaning in every detail, we might try to argue that this door motif is part of a wider visual pattern throughout the film that focuses on boundaries and barriers: from the wind-ravaged fence along the sand dune to the white picket fences along Amity's Main Street, or from the rail that secures Quint in the Orca's pulpit to the bars of the shark cage that fail to protect Hooper from the vandalising shark.
