I have planted beans in approximately 10 inch (25 cm) pot with potting soil. I also water the pot regularly. The beans have grown well and started to flower but the leaves are turning yellowish. Is this some kind of disease or is the plant dying for some reason?
I think you might have a leafhopper infestation - they cause these speckled, greyish markings on the leaves, but usually, leaves start to yellow and then go brown and dry, and I'm not seeing any like that in your image, though you do describe yellowing and drying.
Reference here, further down in the link http://www.offthegridnews.com/survival-gardening-2/top-17-problems-all-green-bean-growers-must-overcome/, unfortunately with no images. If it is leafhopper, insecticidal soap spray should take care of it, but you may need to repeat treat - row covers are a good way of preventing the problem, but your plants aren't in an ideal situation for using those.