feat: add ability to increase the max parallelism when deploying a given process type

docs/deployment/schedulers/docker-local.md

@@ -37,9 +37,9 @@ Once set, you may re-enable it by setting a blank value for `disable-chown`:
 dokku scheduler-docker-local:set node-js-app disable-chown
 ```
 
-### Deploying Process Types in parallel
+### Deploying Process Types in Parallel
 
-> New as of 25.5
+> New as of 0.25.5
 
 By default, Dokku deploys an app's processes one-by-one in order, with the `web` process being deployed first. Deployment parallelism may be achieved by setting the `parallel-schedule-count` property, which defaults to `1`. Increasing this number increases the number of process types that may be deployed in parallel (with the web process being the exception).
 
@@ -60,6 +60,37 @@ Container scheduling output is shown in the order it is received, and thus may b
 
 Note that increasing the value of `parallel-schedule-count` may significantly impact CPU utilization on your host as your app containers - and their respective processes - start up. Setting a value higher than the number of available CPUs is discouraged. It is recommended that users carefully set this value so as not to overburden their server.
 
+#### Increasing parallelism within a process deploy
+
+> New as of 0.26.0
+
+By default, Dokku will deploy one instance of a given process type at a time. This can be increased by customizing the `app.json` `formation` key to include a `max_parallel` key for the given process type.
+
+An `app.json` file can be committed to the root of the pushed app repository, and must be within the built image artifact in the image's working directory as shown below.
+
+- Buildpacks: `/app/app.json`
+- Dockerfile: `WORKDIR/app.json` or `/app.json` (if no working directory specified)
+- Docker Image: `WORKDIR/app.json` or `/app.json` (if no working directory specified)
+
+The `formation` key should be specified as follows in the `app.json` file:
+
+```Procfile
+{
+  "formation": {
+    "web": {
+      "max_parallel": 1
+    },
+    "worker": {
+      "max_parallel": 4
+    }
+  }
+}
+```
+
+Omitting or removing the entry will result in parallelism for that process type to return to 1 entry at a time. This can be combined with the  `parallel-schedule-count` property to speed up deployments.
+
+Note that increasing the value of `max_parallel` may significantly impact CPU utilization on your host as your app containers - and their respective processes - start up. Setting a value higher than the number of available CPUs is discouraged. It is recommended that users carefully set this value so as not to overburden their server.
+
 ## Implemented Triggers
 
 This plugin implements various functionality through `plugn` triggers to integrate with Docker for running apps on a single server. The following functionality is supported by the `scheduler-docker-local` plugin.