What does this lyric mean; 'Wearing a face that she keeps in a jar by the door' ?

Paul McCartney has said its in reference to a lonely old lady wearing cold cream to make herself look younger. 

A face in a jar usually refers to makeup.

Foreshadowing the coronavirus mask 

Another genius lyric by the Beatles.

Ever heard someone say they need time to put their face on?  Referring to their makeup.  It means she putting on makeup.  Kept by the door to me means anytime she's in public.  Her door being the barrier between home alone or out in public

People act cheerful when they are not.  They put happy expressions on their faces for the outside world. In this song, taken literally, Rigby, an unhappy woman, might go to the door, take the happy face from the jar and put it on, then answer the door. When the business was concluded and the person at the door left, back in the jar the face would go.

I always imagined that they’re referring to putting on makeup, or putting on an act. But I’m not sure 

It means that although she is lonely and unhappy

she always 'puts a smile on her face' when she goes out.

She is 'putting on an act' that he's happy.

She uses a lot of makeup.