auth: fix prepare_args passing

Containerfile

@@ -61,7 +61,7 @@ FROM docker.io/library/debian:bookworm-20230904-slim AS shuttle-auth
 ARG CARGO_PROFILE
 ARG prepare_args
 COPY auth/prepare.sh /prepare.sh
-RUN /prepare.sh "${prepare_args}}"
+RUN /prepare.sh "${prepare_args}"
 COPY --from=chef-builder /build/target/${CARGO_PROFILE}/shuttle-auth /usr/local/bin
 ENTRYPOINT ["/usr/local/bin/shuttle-auth"]
 FROM shuttle-auth AS shuttle-auth-dev