MAINT: added an explicit download of rustfmt for the specific toolchain

Author: AvishaiW

dockerfile/Dockerfile

@@ -12,10 +12,9 @@ RUN apt-get update && \
     apt-get install -y --no-install-recommends libzmq3-dev clang \
     && rm -rf /var/lib/apt/lists/*
 
-ENV RUSTUP_TOOLCHAIN=nightly-2019-05-22
-
 RUN /root/.cargo/bin/rustup target add wasm32-unknown-unknown && \
-    /root/.cargo/bin/rustup component add rustfmt && \
+    # remove the toolchain flag when we update to a new nightly version.
+    /root/.cargo/bin/rustup component add rustfmt --toolchain nightly-2019-05-22-x86_64-unknown-linux-gnu && \
     /root/.cargo/bin/cargo install bindgen cargo-audit && \
     rm -rf /root/.cargo/registry && rm -rf /root/.cargo/git