The main cost is less clarity: the build steps aren't isolated anymore, so it's harder to pinpoint issues. There's also obvious risk of my script not interpreting all options correctly.

Actually, I've just disabled VOLUME statement in the script, as it seems to be a no-op in Docker. Only trace it leaves in the image is setting image's command to '/bin/sh -c "#(nop) VOLUME [\"/data\"]"'.