Pin DOKKU_SYSTEM_USER and DOKKU_SYSTEM_GROUP to the current process user in test setup so PropertyListWrite succeeds in CI Docker where the dokku group does not exist.
Add buildpacks:set --replace so callers can replace an app's complete ordered buildpack list in one command while preserving existing single-buildpack and --index behavior.
Closes#8802
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.
