feat: add support for "none" scope

docs/arguments.md

@@ -359,6 +359,11 @@ Environment Variable: WATCHTOWER_HTTP_API_PERIODIC_POLLS
 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.dev/watchtower/running-multiple-instances).
 
+!!! note "Filter by lack of scope"
+    If you want other instances of watchtower to ignore the scoped containers, set this argument to `none`.
+    When omitted, watchtower will update all containers regardless of scope.
+
+
 ```text
             Argument: --scope
 Environment Variable: WATCHTOWER_SCOPE

docs/running-multiple-instances.md

@@ -1,27 +1,41 @@
 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;
+!!! note
+    - 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;
+    - Supplying `none` as the scope will treat `com.centurylinklabs.watchtower.scope=none`, `com.centurylinklabs.watchtower.scope=` and the lack of a `com.centurylinklabs.watchtower.scope` label as the scope `none`. This effectly enables you to run both scoped and unscoped watchtower instances on the same machine.
 
-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).
+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:
 
 ```yaml
 version: '3'
 
 services:
-  app-monitored-by-watchtower:
+  app-with-scope:
     image: myapps/monitored-by-watchtower
-    labels:
-      - "com.centurylinklabs.watchtower.scope=myscope"
+    labels: [ "com.centurylinklabs.watchtower.scope=myscope" ]
 
-  watchtower:
+  scoped-watchtower:
     image: containrrr/watchtower
-    volumes:
-      - /var/run/docker.sock:/var/run/docker.sock
+    volumes: [ "/var/run/docker.sock:/var/run/docker.sock" ]
     command: --interval 30 --scope myscope
-    labels:
-      - "com.centurylinklabs.watchtower.scope=myscope"
+    labels: [ "com.centurylinklabs.watchtower.scope=myscope" ] 
+
+  unscoped-app-a:
+    image: myapps/app-a
+
+  unscoped-app-b:
+    image: myapps/app-b
+    labels: [ "com.centurylinklabs.watchtower.scope=none" ]
+
+  unscoped-app-c:
+    image: myapps/app-b
+    labels: [ "com.centurylinklabs.watchtower.scope=" ]
+
+  unscoped-watchtower:
+    image: containrrr/watchtower
+    volumes: [ "/var/run/docker.sock:/var/run/docker.sock" ]
+    command: --interval 30 --scope none
 ```

pkg/filters/filters.go

@@ -86,13 +86,14 @@ func FilterByDisabledLabel(baseFilter t.Filter) t.Filter {
 
 // FilterByScope returns all containers that belongs to a specific scope
 func FilterByScope(scope string, baseFilter t.Filter) t.Filter {
-	if scope == "" {
-		return baseFilter
-	}
-
 	return func(c t.FilterableContainer) bool {
-		containerScope, ok := c.Scope()
-		if ok && containerScope == scope {
+		containerScope, containerHasScope := c.Scope()
+
+		if !containerHasScope || containerScope == "" {
+			containerScope = "none"
+		}
+
+		if containerScope == scope {
 			return baseFilter(c)
 		}
 
@@ -152,7 +153,13 @@ func BuildFilter(names []string, disableNames []string, enableLabel bool, scope
 		filter = FilterByEnableLabel(filter)
 		sb.WriteString("using enable label, ")
 	}
-	if scope != "" {
+
+	if scope == "none" {
+		// If a scope has explicitly defined as "none", containers should only be considered
+		// if they do not have a scope defined, or if it's explicitly set to "none".
+		filter = FilterByScope(scope, filter)
+		sb.WriteString(`without a scope, "`)
+	} else if scope != "" {
 		// If a scope has been defined, containers should only be considered
 		// if the scope is specifically set.
 		filter = FilterByScope(scope, filter)

pkg/filters/filters_test.go

@@ -111,6 +111,33 @@ func TestFilterByScope(t *testing.T) {
 	container.AssertExpectations(t)
 }
 
+func TestFilterByNoneScope(t *testing.T) {
+	scope := "none"
+
+	filter := FilterByScope(scope, NoFilter)
+	assert.NotNil(t, filter)
+
+	container := new(mocks.FilterableContainer)
+	container.On("Scope").Return("anyscope", true)
+	assert.False(t, filter(container))
+	container.AssertExpectations(t)
+
+	container = new(mocks.FilterableContainer)
+	container.On("Scope").Return("", false)
+	assert.True(t, filter(container))
+	container.AssertExpectations(t)
+
+	container = new(mocks.FilterableContainer)
+	container.On("Scope").Return("", true)
+	assert.True(t, filter(container))
+	container.AssertExpectations(t)
+
+	container = new(mocks.FilterableContainer)
+	container.On("Scope").Return("none", true)
+	assert.True(t, filter(container))
+	container.AssertExpectations(t)
+}
+
 func TestFilterByDisabledLabel(t *testing.T) {
 	filter := FilterByDisabledLabel(NoFilter)
 	assert.NotNil(t, filter)