Allow to create a container without volume
Children images expected some issues like the container set up stuff, but
in fine, it's discarded and our version supersedes their local changes.
Furthermore, volumes don't really make sense in this scenario, as children
image has full control to set up the container. We offer a version without
volume, to ease children image creation.
You can expect the :novolume tag to match the latest tag PHP and nginx
versions, as plans include to rebase this branch continously.