I’m also running into this issue. The container has not changed at all, but now tests that previously passed are failing due to the XVFB error.
Last successful build: Build-agent version 0.0.3763-62d8c9a (2017-07-25T13:09:43+0000)
First failure: Build-agent version 0.0.3793-f06cbf5 (2017-07-26T14:29:53+0000)