Steiger: OCI-native builds for Docker, Bazel, and Nix with direct registry push

We built Steiger after getting frustrated with Skaffold's performance in our Bazel-heavy polyglot monorepo. The main pain points were:

The TAR bottleneck: Skaffold forces Bazel to export OCI images as TAR files, then imports them back into Docker. This is slow and wasteful

Cache invalidation: Skaffold's custom caching layer often conflicts with the sophisticated caching that build systems like Bazel and Nix already provide.

Currently supports:

Docker BuildKit: Uses docker-container driver, manages builder instances

Bazel: Direct OCI layout consumption, skips TAR export entirely

Nix: Works with flake outputs that produce OCI images

Ko: Native Go container builds

Still early days - we're planning file watching for dev mode and (basic) Helm deployment just landed!