Specifying a volume at `/app` (in the Dockerfile) makes it difficult to
use this image as a base image for derived images. Due to this, we have
removed the `VOLUME` instruction that mounts `/app` as a volume.
The `/app` path continues to function like it did before. As a result if
you want to mount your php application into the container, mount your
application source at `/app` using `-v
/path/on/application/source:/app`.