message: Also remove /usr/local/cargo/registry/src/ after building
  Should make the image smaller.
diff: @@ -7,7 +7,8 @@ COPY . /opt/tarpaulin/
  RUN cd /opt/tarpaulin/ && \
  cargo install && \
  rm -rf /opt/tarpaulin/ && \
  - rm -rf /usr/local/cargo/registry/cache/
  + rm -rf /usr/local/cargo/registry/cache/ && \
  + rm -rf /usr/local/cargo/registry/src/
  WORKDIR /volume
  