compose yaml mounts don't override conflicting metadata mounts

When you build an image using devcontainers with mount `/pathA:/pathA` and then reference that image in a compose file, then using that compose file in a separate devcontainer.json with mount `/pathB:/pathA`, the mount in compose.yaml doesn't take precedence over the mount from the metadata, and the resulting container has the (incorrect) mount `/pathA:/pathA`.

The [spec](https://containers.dev/implementors/spec/#merge-logic) for `mounts` says:

> Collected list of all mountpoints. Conflicts: Last source wins.

Which isn't the case. I created a minimally reproducible example https://github.com/Achllle/repro_devc_metadata with instructions and logs.