refactor: move CHECKS file extraction to the beginning of the deploy

This makes deploys faster for any that are built from source as the image artifact isn't involved in those cases while also unifying how the file is handled.

Refs #2760

docs/deployment/zero-downtime-deploys.md

@@ -139,15 +139,13 @@ If your application needs a longer period to boot up - perhaps to load data into
 
 Checks are run against the detected `web` process from your application's `Procfile`. For non-web processes, Dokku will fallback to the aforementioned process uptime check.
 
-To specify checks, add a `CHECKS` file to the root of your project directory. The `CHECKS` file should be plain text and may contain:
+For deploys via the `git:from-image` and `git:load-image` commands, the `CHECKS` file is extracted from the configured `WORKDIR` property of the image. For all other deploys - git push, `git:from-archive`, `git:sync` - will have the `CHECKS` extracted directly from the source code. The filename in both cases is `CHECKS` and cannot be modified. The `CHECKS` file should be plain text and may contain:
 
 - check instructions
 - settings (NAME=VALUE)
 - comments (lines starting with #)
 - empty lines
 
-> For Dockerfile and Docker Image based deploys, the file *must* be in the `WORKDIR` directory of the built image. `/app` is used by default as the root container directory for buildpack-based deploys.
-
 ### Check instructions
 
 The format of a check instruction is a path or relative URL, optionally followed by the expected content: