Add file download to node example

[aeaton-overleaf]

• Use docker-compose.yaml to start the fake-gcs service (copied from the dotnet example, with the addition of a build path and a volumes mount to load the example data).
• Add an example of downloading a file from a bucket.

The download example is currently failing when used with @google-cloud/storage > v5.8.1 unless validation: false is set, which isn't ideal - it seems that fake-gcs needs to be setting a x-goog-hash header on the response.

I've added a line of code that sets X-Goog-Hash on the download response, but I'm not sure when either of obj.Crc32c or obj.Md5Hash are guaranteed to be set on the object, so someone with more knowledge of the server will need to finish this off.

Closes #529.

[fsouza]

Thanks for contributing! I'll push a release once this is merged.

[fsouza]

I'll fix the dotnet issue on master.