Pre-set LOCAL_DOMAIN in the tini test so mastodon's required env var
resolves, and replace unsupported `generator: "echo 5"` with `value: "5"`
in the nodejs-express and dockerfile fixtures so WEB_CONCURRENCY no
longer aborts deploys via the new app.json env processor.
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.
