Buildpack update instructions.md

Compiling Vowpal Wabbit to update this buildpack

These instructions are valid, as of April 2020 to compile vw version 8.8.1 for Ubuntu

1. Use the Heroku-18 stack: docker run -it heroku/heroku:18 /bin/bash.
2. Update apt: apt update -y.
3. Install the vw dependencies by following these instructions for Ubuntu.
   • No need to install Pip, or other optional dependencies
4. Install boost_thread, since it's not in the installation instructions but is stilll needed: apt install -y libboost-thread-dev
5. Because the dynamic zlib library (.so) is found instead of the static one, just run the following to force the use of the static library.
   • rm /usr/lib/x86_64-linux-gnu/libz.so
   • ln -s /usr/lib/x86_64-linux-gnu/libz.a /usr/lib/x86_64-linux-gnu/libz.so
6. Follow instructions here for building on Linux.
7. Compilation has succeeded. To copy the binary from the Docker container,
   • Run docker ps to list active containers
   • Run docker cp <container-id>:/vowpal_wabbit/build/vowpalwabbit ~/Downloads