feat: add a subcommand for retrieving failed app deploy logs

This can be useful in cases where a developer wishes to see what error messages are available for a failed deploy, but does not wish to spelunk around `docker ps -a` output.

Note that the logs are not made available once a further deploy is triggered, or after the containers have been garbage collected.

docs/deployment/logs.md

@@ -1,19 +1,21 @@
 # Log Management
 
 ```
-logs <app>           # display recent log output
+logs <app> [-h] [-t] [-n num] [-q] [-p process]  # Display recent log output
+logs:failed <app>                                # Shows the last failed deploy logs for an app
 ```
 
 ## Usage
 
-### Application Logs
-You can easily get logs of an application using the `logs` command:
+### App Logs
+
+You can easily get logs of an app using the `logs` command:
 
 ```shell
 dokku logs node-js-app
 ```
 
-## Behavioral modifiers
+#### Behavioral modifiers
 
 Dokku also supports certain command-line arguments that augment the `log` command's behavior.
 
@@ -26,8 +28,19 @@ Dokku also supports certain command-line arguments that augment the `log` comman
 
 You can use these modifiers as follows:
 
-
 ```shell
 dokku logs node-js-app -t -p web
 ```
 will show logs continually from the web process.
+
+### Failed Deploy Logs
+
+> Warning: The default `docker-local` scheduler will "store" these until the next deploy or until the old containers are garbage collected - whichever runs first. If you require the logs beyond this point in time, please ship the logs to a centralized log server.
+
+In some cases, it may be useful to retrieve the logs from a previously failed deploy.
+
+You can retrieve these logs by using the `logs:failed` command
+
+```shell
+dokku logs:failed node-js-app
+```