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/arguments.md

@@ -228,6 +228,16 @@ Environment Variable: WATCHTOWER_HTTP_API_TOKEN
              Default: -
 ``` 
 
+## Filter by scope
+Update containers that have a `com.centurylinklabs.watchtower.scope` label set with the same value as the given argument. This enables [running multiple instances](https://containrrr.github.io/watchtower/running-multiple-instances).
+
+```
+            Argument: --scope
+Environment Variable: WATCHTOWER_SCOPE
+                Type: String
+             Default: -
+``` 
+
 ## Scheduling
 [Cron expression](https://pkg.go.dev/github.com/robfig/cron@v1.2.0?tab=doc#hdr-CRON_Expression_Format) in 6 fields (rather than the traditional 5) which defines when and how often to check for new images. Either `--interval` or the schedule expression 
 can be defined, but not both. An example: `--schedule "0 0 4 * * *"`

docs/container-selection.md

@@ -23,3 +23,9 @@ Or, it can be specified as part of the `docker run` command line:
 ```bash
 docker run -d --label=com.centurylinklabs.watchtower.enable=true someimage
 ```
+
+If you wish to create a monitoring scope, you will need to [run multiple instances and set a scope for each of them](https://containrrr.github.io/watchtower/running-multiple-instances).
+
+Watchtower filters running containers by testing them against each configured criteria. A container is monitored if all criteria are met. For example:
+- If a container's name is on the monitoring name list (not empty `--name` argument) but it is not enabled (_centurylinklabs.watchtower.enable=false_), it won't be monitored;
+- If a container's name is not on the monitoring name list (not empty `--name` argument), even if it is enabled (_centurylinklabs.watchtower.enable=true_ and `--label-enable` flag is set), it won't be monitored;

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"
+```