These good people cry “how bright their frail deeds might have danced in a green bay.” what does this mean?

This line comes from Dylan Thomas's poem "Do not go gentle into that good night". There, the speaker admonishes his father against dying peacefully, instead telling him that he should "rage, rage against the dying of the light". He presents several examples of people who do not die in peace, but instead rage against their mortality. One of those examples are "good men" who, instead of resigning themselves to their fate, think about other things they might have done, and how those deeds would have given them a better reputation. Thus, they rage against the dying of the light.