docs: add initial registry docs

docs/deployment/registry-management.md

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+# Registry Management
+
+> New as of 0.25.0
+
+```
+registry:login [--password-stdin] <server> <username> [<password>] # Login to a docker registry
+registry:report [<app>] [<flag>]                                   # Displays a registry report for one or more apps
+registry:set <app> <key> (<value>)                                 # Set or clear a registry property for an app
+```
+
+The registry plugin enables interacting with remote registries, which is useful when either deploying images via `git:from-image` or when interacting with custom schedulers to deploy built image artifacts.
+
+## Usage
+
+### Logging into a registry
+
+The `registry:login` command can be used to log into a docker registry. The following are examples for logging into various common registries:
+
+```shell
+# hub.docker.com
+dokku registry:login docker.io $USERNAME $PASSWORD
+
+# digitalocean
+# the username and password are both defined as the same api token
+dokku registry:login registry.digitalocean.com $DIGITALOCEAN_API_TOKEN $DIGITALOCEAN_API_TOKEN
+
+# github container registry
+# see the following link for information on retrieving a personal access token
+#   https://docs.github.com/en/packages/guides/pushing-and-pulling-docker-images#authenticating-to-github-container-registry
+dokku registry:login ghcr.io $USERNAME $REGISTRY_PAT_TOKEN
+
+# quay
+# a robot user may be used to login
+dokku registry:login quay.io $USERNAME $PASSWORD
+```
+
+For security reasons, the password may also be specified as stdin by specifying the `--password-stdin` flag. This is supported regardless of the registry being logged into.
+
+```shell
+echo "$PASSWORD" | dokku registry:login docker.io $USERNAME
+```
+
+For certain Docker registries - such as Amazon ECR or Google's GCR registries - users may instead wish to use a docker credential helper to automatically authenticate against a server; please see the documentation regarding the credential helper in question for further setup instructions.
+
+### Pushing images on build
+
+To push the image on release, set the `push-on-release` property to `true` via the `registry:set` command. The default value for this property is `false`.
+
+```shell
+dokku registry:set node-js-app push-on-release true
+```
+
+This property can be set for a single app or globally via the `--global` flag. When set globally, the app-specific value will always overide the global value. The default global value for this property is `false`.
+
+```shell
+dokku registry:set --global push-on-release true
+```
+
+Setting the property value to an empty string will reset the value to the system default. Resetting the value can be done per app or globally.
+
+```shell
+# per-app
+dokku registry:set node-js-app push-on-release
+
+# globally
+dokku registry:set --global push-on-release
+```