This commit makes several changes to the dockerfile:
- Add go.mod and go.sum in a seperate stage, subsequently calling `go
mod download` to make it cache dependencies and speed up builds
- Use ubuntu:latest (28MB larger) instead of scratch, makes the image a
lot easier to debug (e.g. it has a shell and a package manager)
- Change ENTRYPOINT to CMD, this makes the behaviour of the image
slightly different from a CLI perspective, but makes interacting with
the image from code, docker-compose and kubernetes easier.
