Use rustup from base image

Dockerfile

@@ -7,9 +7,6 @@ RUN set -eux; \
     apk add bash curl git tar openssh; \
     curl --silent -L https://github.com/EmbarkStudios/cargo-deny/releases/download/$deny_version/cargo-deny-$deny_version-x86_64-unknown-linux-musl.tar.gz | tar -xzv -C /usr/bin --strip-components=1;
 
-# Ensure rustup is up to date.
-RUN bash -c "sh <(curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs) -y"
-
 COPY entrypoint.sh /entrypoint.sh
 
 ENTRYPOINT ["/entrypoint.sh"]