The buildpacks entry in tests/apps/dockerfile/app.json caused
the post-extract trigger to write a .buildpacks file during
Dockerfile-based deploys, breaking builder detection. Remove
the entry and add Go unit tests for getBuildpacks/validBuildpackURL,
a bats integration test for deploying with app.json buildpacks,
and documentation for the new feature.
Some files - those maintained by external organizations - have a very light treatment and solely point to the upstream documentation to reduce any issues creating examples/documentation for them that may differ in the future.
Closes#7315