* Updates bob-builder to a version that clears out the previous
  build directory, which prevents the stale files seen in #379.
* Adds a `buildenv-heroku-16` Make command to simplify the
  building and use of the Heroku-16 binary build environment.
* Sets `S3_BUCKET` and `S3_PREFIX` in the Docker image, to save
  having to do so manually each time.
* Removes the duplication of the bob-builder dependency between
  `Dockerfile` and `requirements.txt`.
* Disables the pip version check during Docker build, to avoid the
  noisy stderr warning about Ubuntu 16.04 pip being older (v8.1.1).
* Adds a `.dockerignore` to speed up the Docker build, by reducing
  the build context transferred to the daemon from 60MB to 2MB.
* Applies some Dockerfile best practices like disabling/removing
  cached files (since layer invalidation makes caching pointless).