I've got some vague recollection (and anyones welcome to jump in here) that, back in the 1800s, miners or railway workers (many of whom were Chinese) carried buckets with which to eat their lunch. Might be where this notion came from.

*Edit* Correction: I found a thread from about 5 years ago that mentions "Bento" boxes that are apparently a Japanese thing. There are a bunch of them on ebay, mostly plastic and decorated.

The thread's here for the curious or bored.