The previous lever for manipulating this was the DOKKU_IMAGE environment variable. While this is all well and good, it only works for CNB and is yet another `DOKKU_*` environment variable that need not exist.
While this does not add support for manipulating the CNB stack just yet, the groundwork is set for the future.
Closes#4306
This unifies all the report-handling code so that plugins only need to worry about the data they wish to represent, and not the logic of actually returning it.
