openapi-codegen: improve build and CI

[svix-jbrown]

Many changes:

• Add prek / pre-commit rules
• Pin github actions by SHA
• Use docker/github-builder instead of doing it by hand
• Move all of the platform-specific logic into the Dockerfile instead of the Github Actions so you can just do docker build on any platform and it works
• Bump to alpine 3.24 from 3.21
• Add dependabot
• Run the full build in CI (but don't push it!)