feat: add support for non-web healthchecks via app.json

This change converts existing CHECKS files into a healthchecks key that is understood by 'docker-container-healthchecker'. This tool supports a number of different types of container healthchecks - command, http, uptime - and can perform healthchecks against non-web processes.

The use of the old CHECKS file is now deprecated, and will be removed in the next minor version. Users can use the 'docker-container-healthchecker' to convert existing CHECKS files to the new format automatically.

Closes #2760

docs/development/plugin-triggers.md

@@ -104,6 +104,21 @@ set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
 # TODO
 ```
 
+### `app-json-get-content`
+
+- Description: Outputs the contents of the app-json file, if any
+- Invoked by: Deployment checks
+- Arguments: `$APP`
+- Example:
+
+```shell
+#!/usr/bin/env bash
+
+set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
+
+# TODO
+```
+
 ### `app-maybe-create`
 
 - Description: Creates an app (gated by whether this is globally enabled or not)