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
Moving verification of input to the earliest place is the best way to decrease the need to validate this further downstream.
Additionally, some of the existing checks were.... lacking :)
This change refactors the compiled golang plugins into 3 distinct binaries:
- commands: already existing
- subcommands/subcommands: entrypoint into all subcommands
- triggers: entrypoint into all triggers
It then further symlinks triggers and subcommands to the built binaries. This results in both a much faster build process as well as smaller package size.
