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

README.md

@@ -93,6 +93,7 @@ docker run --rm v2tec/watchtower --help
 * `--interval, -i` Poll interval (in seconds). This value controls how frequently watchtower will poll for new images. Defaults to 300 seconds (5 minutes).
 * `--schedule, -s` [Cron expression](https://godoc.org/github.com/robfig/cron#hdr-CRON_Expression_Format) which defines when and how often to check for new images. Either `--interval` or the schedule expression could be defined, but not both.
 * `--no-pull` Do not pull new images. When this flag is specified, watchtower will not attempt to pull new images from the registry. Instead it will only monitor the local image cache for changes. Use this option if you are building new images directly on the Docker host without pushing them to a registry.
+* `--label-enable` Watch containers where the `com.centurylinklabs.watchtower.enable` label is set to true.
 * `--cleanup` Remove old images after updating. When this flag is specified, watchtower will remove the old image after restarting a container with a new image. Use this option to prevent the accumulation of orphaned images on your system as containers are updated.
 * `--tlsverify` Use TLS when connecting to the Docker socket and verify the server's certificate.
 * `--debug` Enable debug mode. When this option is specified you'll see more verbose logging in the watchtower log file.
@@ -123,6 +124,22 @@ Or, it can be specified as part of the `docker run` command line:
 docker run -d --label=com.centurylinklabs.watchtower.stop-signal=SIGHUP someimage
 ```
 
+## Selectively Watching Containers
+
+By default, watchtower will watch all containers.
+However, sometimes only some containers should be updated.
+If you need to selectively watch containers, pass the --label-enable flag on startup and set the *com.centurylinklabs.watchtower.enable* label with a value of true for the containers you want to watch.
+
+```docker
+LABEL com.centurylinklabs.watchtower.enable="true"
+```
+
+Or, it can be specified as part of the `docker run` command line:
+
+```bash
+docker run -d --label=com.centurylinklabs.watchtower.enable=true someimage
+```
+
 ## Remote Hosts
 
 By default, watchtower is set-up to monitor the local Docker daemon (the same daemon running the watchtower container itself). However, it is possible to configure watchtower to monitor a remote Docker endpoint. When starting the watchtower container you can specify a remote Docker endpoint with either the `--host` flag or the `DOCKER_HOST` environment variable:

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
+	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 {

main.go

@@ -79,6 +79,11 @@ func main() {
 			Usage:  "use TLS and verify the remote",
 			EnvVar: "DOCKER_TLS_VERIFY",
 		},
+		cli.BoolFlag{
+			Name:   "label-enable",
+			Usage:  "watch containers where the com.centurylinklabs.watchtower.enable label is true",
+			EnvVar: "WATCHTOWER_LABEL_ENABLE",
+		},
 		cli.BoolFlag{
 			Name:  "debug",
 			Usage: "enable debug mode with verbose logging",
@@ -146,7 +151,7 @@ func before(c *cli.Context) error {
 		return err
 	}
 
-	client = container.NewClient(!c.GlobalBool("no-pull"))
+	client = container.NewClient(!c.GlobalBool("no-pull"), c.GlobalBool("label-enable"))
 	notifier = notifications.NewNotifier(c)
 
 	return nil