For deploying a slug you have pre-built. A slug on gigalixir is often a release tarball generated by running
MIX_ENV=prod mix release
But it could be any slug containing any sort of app, including non-elixir apps. For example, you could do something like this to create a slug that does not use releases.
tar --dereference -z -cf app.tar.gz --exclude='.git' -C .
You then upload the slug somewhere and point to it in a slug_buildpack.config file that looks like this
slug_url=https://example.com/app.tar.gz