feat: Pull invalid nginx configuration when the nginx configs fail to validate

- expose an nginx:validate command that can be used to show validation information
- properly reference the invalid nginx config, rather than tell the user that another app's nginx config is invalid when deploying their own app
- allow a user to cleanup bad nginx config files out of band when they are blocking a deploy _without_ requiring knowing where that nginx config is

Note that this may have issues with generated nginx.conf files that depend on other apps or other parts of the nginx config. For now, this is acceptable, though it is something that needs to be field tested.

Closes #3162

docs/configuration/nginx.md

@@ -6,6 +6,7 @@ Dokku uses nginx as its server for routing requests to specific applications. By
 nginx:access-logs <app> [-t]             # Show the nginx access logs for an application (-t follows)
 nginx:build-config <app>                 # (Re)builds nginx config for given app
 nginx:error-logs <app> [-t]              # Show the nginx error logs for an application (-t follows)
+nginx:validate [<app>] [--clean]         # Validates and optionally cleans up invalid nginx configurations
 ```
 
 ## Checking access logs
@@ -44,6 +45,30 @@ In certain cases, your app nginx configs may drift from the correct config for y
 dokku nginx:build-config node-js-app
 ```
 
+## Validating nginx configs
+
+It may be desired to validate an nginx config outside of the deployment process. To do so, run the `nginx:validate` command. With no arguments, this will validate all app nginx configs, one at a time. A minimal wrapper nginx config is generated for each app's nginx config, upon which `nginx -t` will be run.
+
+```shell
+dokku nginx:validate
+```
+
+As app nginx configs are actually executed within a shared context, it is possible for an individual config to be invalid when being validated standalone but _also_ be valid within the global server context. As such, the exit code for the `nginx:validate` command is the exit code of `nginx -t` against the server's real nginx config.
+
+The `nginx:validate` command also takes an optional `--clean` flag. If specified, invalid nginx configs will be removed.
+
+> Warning: Invalid app nginx config's will be removed _even if_ the config is valid in the global server context.
+
+```shell
+dokku nginx:validate --clean
+```
+
+The `--clean` flag may also be specified for a given app:
+
+```shell
+dokku nginx:validate node-js-app --clean
+```
+
 ## Customizing the nginx configuration
 
 > New as of 0.5.0