Rather than hardcode two builders, allow builders to specify a `builder-detect` trigger. This trigger can be used to specify if the builder should or should not be used for an application. Each builder takes stdin and can decide if it wants to emit it or emit it's own image source type.
If the final value is empty, then Dokku will default to herokuish (and cnb once that is stable). In addition, a future change may allow users to manually specify a builder in the case they wish to override the choice selected by Dokku.
This change enables users to build custom builder plugins and have those plugins used for building an image asset. By way of example, an enterprising user could create a `builder-lambda` based on lambci, and then pair this with a scheduler plugin that updates a lambda function on AWS. Alternatively, a user might decide they wish to place their Dockerfile in a specific directory for their applications - such as an `_infrastructure` directory - and create a plugin to override how that is detected within Dokku.
This standardization makes it somewhat easier to read stacktraces as the command names are all uniform, so it will be slightly easier to scan trace output.
This will allow background processes to cleanup these files if they were unable to be cleaned up during a run of the dokku binary, as is the case for files copied by the docker binary when running in docker.
While I do not agree with _every_ style change, this will force Dokku to have consistent formatting across all shell scripts, which is arguably a Good Thing™.
The command used to reprocess everything is:
```shell
shfmt -l -bn -ci -i 2 -w .
```
This trigger allows you to modify the contents of an application *after* it has been extracted from git/tarball but *before* the image source type is detected. For example, you may wish to autogenerate a Dockerfile based on some other application contents before detecting what type of build process - herokuish or docker file - to use.
