Add Dockerfile template to blueprint

[oliverbarnes]

While using Gerust for https://github.com/mainmatter/reStations, once it came time to release a docker image, we had to create one from scratch.

It'd be handy to have a production-ready Dockerfile in the generated app, saving time for users.

Pavex includes a starter example specific for Rust apps, using cargo-chef to optimize the image build.