Publish .proto somewhere

[ciaranmcnulty]

Description

There's already a step in the build that extracts the .proto service definitions cleanly from this repository

Because it takes a few steps to do this, it would be interesting to share that work by publishing that stage as an image / TAR / Git repo as part of the build

This would aid people experimenting with non-Go versions of the various components.

[tonistiigi]

I don't think we need to publish this, but we can add a bake target for easy extraction with one command (bake also works on remote repositories so you don't even need a checkout).