feat: add support for the Procfile release command

Closes #3136

docs/advanced-usage/deployment-tasks.md

@@ -2,25 +2,52 @@
 
 > New as of 0.5.0
 
-Sometimes you need to run a command on at deployment time, but before an app is completely deployed.
-Common use cases include:
+Sometimes you need to run a command on at deployment time, but before an app is completely deployed. Common use cases include:
 
 * Checking a database is initialized
 * Running database migrations
 * Any commands required to set up the server (e.g. something like a Django `collectstatic`)
 
-## `app.json` and `scripts.dokku`
+To support this, Dokku provides support for a special `release` command within your app's `Procfile`, as well as a special `scripts.dokku` key inside of your app's `app.json` file. Be aware that all commands are run within the context of the built docker image - no commands affect the host unless there are volume mounts attached to your app.
 
-Dokku accomplishes this by using an `app.json` file. The format in use is similar to format of Heroku's [app.json](https://devcenter.heroku.com/articles/app-json-schema).
-However, Dokku currently only supports the nodes `scripts.dokku.predeploy` and `scripts.dokku.postdeploy`.
-For buildpack apps, simply place an `app.json` file in the root of your repository.
-For dockerfile apps, place `app.json` in the configured `WORKDIR` directory; otherwise Dokku defaults to the buildpack app behavior of looking in `/app`.
+Each "phase" has different expectations and limitations:
 
-> NOTE: postdeploy changes are *NOT* committed to the app image.
+- `app.json`: `scripts.dokku.predeploy`
+    - When to use: This should be used if your app does not support arbitrary build commands and you need to make changes to the built image.
+    - Are changes committed to the image at this phase: Yes
+    - Example use-cases
+        - Bundling assets in a slightly different way
+        - Installing a custom package from source or copying a binary into place
+- `app.json`: `scripts.dokku.postdeploy`
+    - When to use: This should be used in conjunction with external systems to signal the completion of your deploy.
+    - Are changes committed to the image at this phase: No
+    - Example use-cases
+        - Notifying slack that your app is deployed
+        - Coordinating traffic routing with a central load balancer
+- `Procfile`: `release`
+    - When to use: This should be used in conjunction with external systems to signal the completion of your app image build.
+    - Are changes committed to the image at this phase: No
+    - Example use-cases
+        - Sending CSS, JS, and other assets from your app’s slug to a CDN or S3 bucket
+        - Priming or invalidating cache stores
+        - Running database migrations
 
-### Example app.json
+Please keep the above in mind when utilizing deployment tasks.
 
-> NOTE: Only the `scripts.dokku.predeploy` and `scripts.dokku.postdeploy` tasks are supported by Dokku at this time. All other fields will be ignored and can be omitted.
+> To execute commands on the host during a release phase, see the [plugin creation documentation](/docs/development/plugin-creation) docs for more information on building your own custom plugin.
+
+## `app.json` deployment tasks
+
+Dokku provides limited support for the `app.json` manifest from Heroku (documentation available [here](https://devcenter.heroku.com/articles/app-json-schema)). The keys available for use with Deployment Tasks are:
+
+- `scripts.dokku.predeploy`: This is run _after_ an app's docker image is built, but _before_ any containers are scheduled. Changes made to your image are committed at this phase.
+- `scripts.dokku.postdeploy`: This is run _after_ an app's containers are scheduled. Changes made to your image are *not* committed at this phase.
+
+For buildpack-based deployments, the location of the `app.json` file should be at the root of your repository. Dockefile-based app deploys should have the `app.json` in the configured `WORKDIR` directory; otherwise Dokku defaults to the buildpack app behavior of looking in `/app`.
+
+> Warning: Any failed `app.json` deployment task will fail the deploy. In the case of either phase, a failure will not affect any running containers.
+
+The following is an example `app.json` file. Please note that only the `scripts.dokku.predeploy` and `scripts.dokku.postdeploy` tasks are supported by Dokku at this time. All other fields will be ignored and can be omitted.
 
 ```json
 {
@@ -32,3 +59,19 @@ For dockerfile apps, place `app.json` in the configured `WORKDIR` directory; oth
   }
 }
 ```
+
+## Procfile Release command
+
+> New as of 0.14.0
+
+The `Procfile` also supports a special `release` command which acts in a similar way to the [Heroku Release Phase](https://devcenter.heroku.com/articles/release-phase). This command is executed _after_ an app's docker image is built, but _before_ any containers are scheduled. This is also run _after_ any command executed by `scripts.dokku.predeploy`.
+
+To use the `release` command, simply add a `release` stanza to your Procfile.
+
+```Procfile
+release: curl https://some.external.api.service.com/deployment?state=built
+```
+
+Unlike the `scripts.dokku.predeploy` command, changes made during by the `release` command are *not* persisted to disk.
+
+> Warning: scaling the release command up will likely result in unspecified issues within your deployment, and is highly discouraged.