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.
When developing Dokku, it is useful to use a real domain to test actual functionality. This is impossible if the domain is hardcoded to dokku.me in testing.
Previously, the shorthand was not properly supported. While this _should_ be the domain of the gliderlabs/herokuish repository, we do it here as a courtesy to Dokku users.
Additionally, support the heroku-community => heroku mapping.
Closes#4452
