Bind-mounting data/kiwix/ (the directory) meant a symlink placed inside it broke: the container has to resolve that symlink itself, in its own filesystem, where the host path it points to doesn't exist. Binding the exact data/kiwix/wikipedia.zim file path instead resolves it on the host at mount time (plain mount(2) semantics), so the real ZIM can live anywhere on the host without duplicating a multi-GB file into this repo's data/ directory.