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

pkg/container/container.go

@@ -90,6 +90,17 @@ func (c Container) Enabled() (bool, bool) {
 	return parsedBool, true
 }
 
+// Scope returns the value of the scope UID label and if the label
+// was set.
+func (c Container) Scope() (string, bool) {
+	rawString, ok := c.getLabelValue(scope)
+	if !ok {
+		return "", false
+	}
+
+	return rawString, true
+}
+
 // Links returns a list containing the names of all the containers to which
 // this container is linked.
 func (c Container) Links() []string {

pkg/container/metadata.go

@@ -6,6 +6,7 @@ const (
 	enableLabel           = "com.centurylinklabs.watchtower.enable"
 	dependsOnLabel        = "com.centurylinklabs.watchtower.depends-on"
 	zodiacLabel           = "com.centurylinklabs.zodiac.original-image"
+	scope                 = "com.centurylinklabs.watchtower.scope"
 	preCheckLabel         = "com.centurylinklabs.watchtower.lifecycle.pre-check"
 	postCheckLabel        = "com.centurylinklabs.watchtower.lifecycle.post-check"
 	preUpdateLabel        = "com.centurylinklabs.watchtower.lifecycle.pre-update"

pkg/container/mocks/FilterableContainer.go

@@ -55,3 +55,27 @@ func (_m *FilterableContainer) Name() string {
 
 	return r0
 }
+
+// Scope provides a mock function with given fields:
+func (_m *FilterableContainer) Scope() (string, bool) {
+	ret := _m.Called()
+
+	var r0 string
+
+	if rf, ok := ret.Get(0).(func() string); ok {
+		r0 = rf()
+	} else {
+		r0 = ret.Get(0).(string)
+	}
+
+	var r1 bool
+
+	if rf, ok := ret.Get(1).(func() bool); ok {
+		r1 = rf()
+	} else {
+		r1 = ret.Get(1).(bool)
+	}
+
+	return r0, r1
+}
+