Nightly image now derives from stable image

ci/docker-rust/Dockerfile

@@ -1,15 +1,19 @@
 FROM rust:1.28
 
-RUN apt update && \
+RUN set -x &&
+    apt update && \
     apt-get install apt-transport-https && \
     echo deb https://apt.buildkite.com/buildkite-agent stable main > /etc/apt/sources.list.d/buildkite-agent.list && \
     apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 32A37959C2FA5C3C99EFBC32A79206696452D198 && \
     apt update && \
     apt install -y \
       buildkite-agent \
+      cmake \
+      llvm-6.0 \
       rsync \
       sudo \
-      cmake \
       && \
     rustup component add rustfmt-preview && \
-    rm -rf /var/lib/apt/lists/*
+    rm -rf /var/lib/apt/lists/* && \
+    rustc --version && \
+    cargo --version