[JENKINS-20772] Fix scrolling

Define a size for the container div that's slightly smaller than the dialog. This way, its new `overflow:scroll` style will be considered, not some ancestor YUI container's `overflow:hidden`.

Tested in Firefox 26 and Safari 6.1 on OS X 10.8.6.

To facilitate manual visual inspection of element sizes, add e.g. the line `$(containerId).style.backgroundColor = "white";` to make the container div stand out.