Use ``busybox`` for Docker-based tests.

Instead of using the ``wordpress`` Docker image for our existing tests,
I am proposing that we switch to using the ``busybox`` image since it is
much smaller and should speed up our tests.

I have not run our tests but I did manually try it using latest build
with success.
Merge pull request #1608 from omaciel/master

Use ``busybox`` for Docker-based tests.