Always use latest rustup when building the container. (#30)

Dockerfile

@@ -4,9 +4,12 @@ ENV deny_version=0.8.4
 
 RUN set -eux; \
     apk update; \
-    apk add curl; \
+    apk add bash curl; \
     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"]