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).

This reverts commit ebded177705e257d8227a836bade85331d56f8d7.
Merge pull request #2750 from bacongobbler/example-dockerfile-http

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