Use NGinx Alpine Docker image to reduce size. (#6171)
Use NGinx Alpine Docker image to reduce size. (#6171)

Summary:
The current Docker image in our tests is above 50MB. This reduces the
size to 8MB and thus the test runtime.
gsantovena/marathon,gsantovena/marathon,mesosphere/marathon,mesosphere/marathon,gsantovena/marathon,gsantovena/marathon,mesosphere/marathon,mesosphere/marathon,gsantovena/marathon,mesosphere/marathon