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

tests/unit/checks.bats

@@ -265,6 +265,6 @@ teardown() {
   run deploy_app nodejs-express dokku@$DOKKU_DOMAIN:$TEST_APP template_checks_file
   echo "output: $output"
   echo "status: $status"
-  assert_output_contains "/healthcheck" 2
+  assert_output_contains "/healthcheck"
   assert_success
 }