Document the repo plugin

[ci skip]

docs/advanced-usage/repository-management.md

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+# Repository Management
+
+> New as of 0.6.0
+
+```
+repo:gc <app>                            # Runs 'git gc --agressive' against the application's repo
+repo:purge-cache <app>                   # Deletes the contents of the build cache stored in the repository
+```
+
+The repository plugin is meant to allow users to perform management commands against a repository.
+
+# Git Garbage Collection
+
+This will run a git gc --agressive against the applications repo. This is performed on the Dokku host, and not within an application container.
+
+```shell
+dokku repo:gc node-js-app
+```
+
+```
+Counting objects: 396, done.
+Delta compression using up to 2 threads.
+Compressing objects: 100% (365/365), done.
+Writing objects: 100% (396/396), done.
+Total 396 (delta 79), reused 315 (delta 0)
+```
+
+## Clearing Application cache
+
+Building containers with buildpacks currently results in a persistent `cache` directory between deploys. If you need to clear this cache directory for any reason, you may do so by running the following shell command:
+
+```shell
+dokku repo:purge-cache node-js-app
+```