Consider using `ko` instead of `docker build`

[`ko`](https://github.com/google/ko) is a tool for building Docker images that are simple containerized Go programs. In my opinion, this is preferable to `docker build` for several reasons:
- Does [reproducible builds](https://reproducible-builds.org/), which are a big plus for supply-chain security
- Native support of multi-arch builds
- Can take advantage of the host's Go module+compiler cache for faster builds. (This is sorta possible with `docker build` using experimental buildkit extensions)

Related to: https://github.com/envoyproxy/gateway/pull/83