Update .dockerignore to include deb file exclusions

[andytinkham]

Now that debify respects .dockerignore files, we should review the .dockerignore file here and make sure it excludes anything we don't want in the deb package (including temporary build artifacts, test reports, etc. that might not exist in the repo pre-build process).

[izgeri]

@andytinkham did you mean to file this in the Ruby CLI?

[andytinkham]

@izgeri I think I put this in the right place... there's a build-deb.sh and a .dockerignore. Do you mean "did I put this here instead of ?" If so, what's the somewhere else you're thinking of?