message: Use rustc version matching minimum Gecko version
  see
  It's ok if we don't exactly track Gecko's rust version, but we shouldn't
  get ahead of it. I think we want 1.41 to use the new lockfile format.
diff: @@ -15,7 +15,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
  ENV RUSTUP_HOME=/usr/local/rustup \
  CARGO_HOME=/usr/local/cargo \
  PATH=/usr/local/cargo/bin:$PATH \
  - RUST_VERSION=1.40.0
  + RUST_VERSION=1.41.1
  RUN set -eux; \
  curl -sSLf "https://static.rust-lang.org/rustup/archive/1.20.2/x86_64-unknown-linux-gnu/rustup-init" -o rustup-init; \
  