Add a method of enabling or disabling containers using labels

Switch command line flag from no-enable to label-enable and simplify logic

Add basic documentation for the --label-enable flag

container/client.go

@@ -37,19 +37,20 @@ type Client interface {
 //  * DOCKER_HOST			the docker-engine host to send api requests to
 //  * DOCKER_TLS_VERIFY		whether to verify tls certificates
 //  * DOCKER_API_VERSION	the minimum docker api version to work with
-func NewClient(pullImages bool) Client {
+func NewClient(pullImages, enableLabel bool) Client {
 	cli, err := dockerclient.NewEnvClient()
 
 	if err != nil {
 		log.Fatalf("Error instantiating Docker client: %s", err)
 	}
 
-	return dockerClient{api: cli, pullImages: pullImages}
+	return dockerClient{api: cli, pullImages: pullImages, enableLabel: enableLabel}
 }
 
 type dockerClient struct {
-	api        *dockerclient.Client
-	pullImages bool
+	api         *dockerclient.Client
+	pullImages  bool
+	enableLabel bool
 }
 
 func (client dockerClient) ListContainers(fn Filter) ([]Container, error) {
@@ -77,6 +78,16 @@ func (client dockerClient) ListContainers(fn Filter) ([]Container, error) {
 		}
 
 		c := Container{containerInfo: &containerInfo, imageInfo: &imageInfo}
+
+		if client.enableLabel {
+			// If label filtering is enabled, containers should only be enabled
+			// if the label is specifically set to true.
+			enabledLabel, ok := c.Enabled()
+			if !ok || !enabledLabel {
+				continue
+			}
+		}
+
 		if fn(c) {
 			cs = append(cs, c)
 		}

container/container.go

@@ -2,6 +2,7 @@ package container
 
 import (
 	"fmt"
+	"strconv"
 	"strings"
 
 	"github.com/docker/docker/api/types"
@@ -11,6 +12,7 @@ import (
 const (
 	watchtowerLabel = "com.centurylinklabs.watchtower"
 	signalLabel     = "com.centurylinklabs.watchtower.stop-signal"
+	enableLabel     = "com.centurylinklabs.watchtower.enable"
 	zodiacLabel     = "com.centurylinklabs.zodiac.original-image"
 )
 
@@ -64,6 +66,22 @@ func (c Container) ImageName() string {
 	return imageName
 }
 
+// Enabled returns the value of the container enabled label and if the label
+// was set.
+func (c Container) Enabled() (bool, bool) {
+	rawBool, ok := c.containerInfo.Config.Labels[enableLabel]
+	if !ok {
+		return false, false
+	}
+
+	parsedBool, err := strconv.ParseBool(rawBool)
+	if err != nil {
+		return false, false
+	}
+
+	return parsedBool, true
+}
+
 // Links returns a list containing the names of all the containers to which
 // this container is linked.
 func (c Container) Links() []string {