diff --git a/ci/docker-rust-nightly/Dockerfile b/ci/docker-rust-nightly/Dockerfile
index 7448983822..1dacf11e64 100644
--- a/ci/docker-rust-nightly/Dockerfile
+++ b/ci/docker-rust-nightly/Dockerfile
@@ -3,4 +3,7 @@ FROM rustlang/rust:nightly
 RUN cargo install --force clippy cargo-cov && \
     echo deb http://ftp.debian.org/debian stretch-backports main >> /etc/apt/sources.list && \
     apt update && \
-    apt install -y llvm-6.0
+    apt install -y \
+      llvm-6.0
+      && \
+    rm -rf /var/lib/apt/lists/*