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

plugins/app-json/Makefile

@@ -1,5 +1,5 @@
 SUBCOMMANDS = subcommands/report subcommands/set
-TRIGGERS = triggers/app-json-process-deploy-parallelism triggers/core-post-deploy triggers/core-post-extract triggers/install triggers/post-app-clone-setup triggers/post-app-rename triggers/post-app-rename-setup triggers/post-create triggers/post-delete triggers/post-deploy triggers/pre-deploy triggers/report
+TRIGGERS = triggers/app-json-process-deploy-parallelism triggers/app-json-get-content triggers/core-post-deploy triggers/core-post-extract triggers/install triggers/post-app-clone-setup triggers/post-app-rename triggers/post-app-rename-setup triggers/post-create triggers/post-delete triggers/post-deploy triggers/pre-deploy triggers/report
 BUILD = commands subcommands triggers
 PLUGIN_NAME = app-json
 

plugins/app-json/src/triggers/triggers.go

@@ -22,6 +22,9 @@ func main() {
 		appName := flag.Arg(0)
 		processType := flag.Arg(1)
 		err = appjson.TriggerAppJSONProcessDeployParallelism(appName, processType)
+	case "app-json-get-content":
+		appName := flag.Arg(0)
+		err = appjson.TriggerAppJSONGetContent(appName)
 	case "core-post-deploy":
 		appName := flag.Arg(0)
 		err = appjson.TriggerCorePostDeploy(appName)

plugins/app-json/triggers.go

@@ -37,6 +37,28 @@ func TriggerAppJSONProcessDeployParallelism(appName string, processType string)
 	return nil
 }
 
+// TriggerAppJSONGetContent outputs the contents of the app-json file, if any
+func TriggerAppJSONGetContent(appName string) error {
+	if !hasAppJSON(appName) {
+		fmt.Print("{}")
+		return nil
+	}
+
+	b, err := os.ReadFile(getProcessSpecificAppJSONPath(appName))
+	if err != nil {
+		return fmt.Errorf("Cannot read app.json file: %v", err)
+	}
+
+	content := strings.TrimSpace(string(b))
+	if content == "" {
+		fmt.Print("{}")
+		return nil
+	}
+
+	fmt.Print(content)
+	return nil
+}
+
 // TriggerCorePostDeploy sets a property to
 // allow the app to be restored on boot
 func TriggerCorePostDeploy(appName string) error {