Reuse containers here for fulcio/rekor proto generation

[loosebazooka]

Proto builder containers here provide

• Access to google, googleapis proto defs
• An update mechanism

The go protobuf container generated here is almost sufficient for generating the rekor(v2)/fulcio code except it needs

• tool: github.com/grpc-ecosystem/grpc-gateway/v2/protoc-gen-grpc-gateway
• tool: github.com/grpc-ecosystem/grpc-gateway/v2/protoc-gen-openapiv2

publish image somewhere

• ghcr?
• AR in sigstore.dev GCP project

Alternatively, just have a separate container image repo for these images (more work -- less obvious value right now?)