Adds scopeUID config to enable multiple instances of Watchtower (#511)

* Adds scopeUID config to enable multiple instances of Watchtower

* Adds tests for multiple instance support with scopeuid

* Adds docs on scope monitoring and multiple instance support

* Adds multiple instances docs to mkdocs config file

* Changes multiple instances check and refactors naming for scope feature

* Applies linter suggestions

* Fixes documentation on Watchtower monitoring scope

docs/running-multiple-instances.md

@@ -0,0 +1,27 @@
+By default, Watchtower will clean up other instances and won't allow multiple instances running on the same Docker host or swarm. It is possible to override this behavior by defining a [scope](https://containrrr.github.io/watchtower/arguments/#filter_by_scope) to each running instance. 
+
+Notice that:
+- Multiple instances can't run with the same scope;
+- An instance without a scope will clean up other running instances, even if they have a defined scope;
+
+To define an instance monitoring scope, use the `--scope` argument or the `WATCHTOWER_SCOPE` environment variable on startup and set the _com.centurylinklabs.watchtower.scope_ label with the same value for the containers you want to include in this instance's scope (including the instance itself).
+
+For example, in a Docker Compose config file:
+
+```json
+version: '3'
+
+services:
+  app-monitored-by-watchtower:
+    image: myapps/monitored-by-watchtower
+    labels:
+      - "com.centurylinklabs.watchtower.scope=myscope"
+
+  watchtower:
+    image: containrrr/watchtower
+    volumes:
+      - /var/run/docker.sock:/var/run/docker.sock
+    command: --interval 30 --scope myscope
+    labels:
+      - "com.centurylinklabs.watchtower.scope=myscope"
+```