Consider using `ko` instead of `docker build`

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, 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