message: Don't crash when installing cargo-update on startup
  cargo-update depends on openssl-sys, which requires libssl-dev.
  I didn't catch this earlier because I had `.rustwide` cached from before
  the dependency was removed.
diff: @@ -63,7 +63,8 @@ RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y \
  ca-certificates \
  build-essential \
  gcc \
  - pkg-config
  + pkg-config \
  + libssl-dev
  RUN mkdir -p /opt/docsrs/prefix
  