Docker: remove `VOLUME` directive (#11997)

The driver for this is to stop Complement complaining about it, but as far as I can tell it was pointless and needed to go away anyway.

I'm a bit unclear about what exactly VOLUME does, but I think what it means is that, if you don't override it with an explicit -v argument, then docker run will create a temporary volume, and copy things into it. The temporary volume is then deleted when the container finishes.

That only sounds useful if your image has something to copy into it (otherwise you may as well just use the default root filesystem), and our image notably doesn't copy anything into /data.

So... this wasn't doing anything, except annoying Complement?