feat: implement build tracking

The DOKKU_PID now never gets overwritten except in the case that DOKKU is executed by the sudo user. If the command ends up executing a deploy, then the pid of the `dokku` owned process - which may have been executed via sudo - will be written to the file lock, allowing future commands to interact with the original process.

Additionally, the new builds plugin can be used to handle killing a build.

docs/deployment/builds-management.md

@@ -0,0 +1,30 @@
+# Build Management
+
+> New as of 0.19.0
+
+```
+builds:cancel <app>                            # Cancel a running build for an app
+builds:list <app>                              # List all running builds
+builds:output <app>                           #  Shows build output
+builds:report [<app>] [<flag>]                 # Displays a build report for one or more apps
+```
+
+## Usage
+
+### Listing running deploys
+
+### Viewing the status of a deploy
+
+### Viewing build output for a deploy
+
+### Canceling a running deploy
+
+It can be useful to kill a deploy if that deploy does not appear to be progressing, is impacting other apps through system resource utilization, or if a successful deploy will result in app errors. To do so, the `builds:cancel` command can be used:
+
+```shell
+dokku builds:cancel node-js-app
+```
+
+This command will send a `QUIT` signal to the Process Group ID of the process handling the deploy, and should terminate all processes within that process tree. Finally, it will unlock the deploy so that a new deploy may be immediately invoked.
+
+> Warning: This may also result in invalid app state depending upon when the app deploy was killed.