re-revert "ref(tests): switch to example-dockerfile-http app"

example-dockerfile-http has switched over to ubuntu:trusty for better
compatibility with apps:run (busybox does not ship with bash). It's not a significant jump like the `busybox` image, but it does shave off about 700MB from the app image and should speed up integration tests.

This reverts commit ebded177705e257d8227a836bade85331d56f8d7.