Very probably Mildew, after all runner beans grow well up stick in containers.
Thing is, I’d doubt if it is really worth it. They need quite a bit of space, so even if you used a large half barrel sized container you’d probably only get 4 or 5 plants in it, and how many beans will that produce? A couple of small feeds maybe. In terms of container costs and time invested in waiting for them to mature, versus returns, I should think they would certainly be the most expensive beans you’ll ever grow.
…..But you can prove me completely wrong of course!!