Feat/lifecycle hooks (#351)

* feat(update): add lifecycle hooks to the update action

* fix(ci): add bash tests for lifecycle-hooks to the ci workflow

* fix(ci): move integration tests to an isolated step

* fix(ci): fix malformed all-contributors json

* fix(ci): disable automatic bash test until we figure out a reasonable way to run it in circleci

pkg/container/metadata.go

@@ -0,0 +1,39 @@
+package container
+
+const (
+	watchtowerLabel = "com.centurylinklabs.watchtower"
+	signalLabel     = "com.centurylinklabs.watchtower.stop-signal"
+	enableLabel     = "com.centurylinklabs.watchtower.enable"
+	zodiacLabel     = "com.centurylinklabs.zodiac.original-image"
+	preUpdateLabel  = "com.centurylinklabs.watchtower.lifecycle.pre-update"
+	postUpdateLabel = "com.centurylinklabs.watchtower.lifecycle.post-update"
+)
+
+// GetLifecyclePreUpdateCommand returns the pre-update command set in the container metadata or an empty string
+func (c Container) GetLifecyclePreUpdateCommand() string {
+	return c.getLabelValueOrEmpty(preUpdateLabel)
+}
+
+// GetLifecyclePostUpdateCommand returns the post-update command set in the container metadata or an empty string
+func (c Container) GetLifecyclePostUpdateCommand() string {
+	return c.getLabelValueOrEmpty(postUpdateLabel)
+}
+
+// ContainsWatchtowerLabel takes a map of labels and values and tells
+// the consumer whether it contains a valid watchtower instance label
+func ContainsWatchtowerLabel(labels map[string]string) bool {
+	val, ok := labels[watchtowerLabel]
+	return ok && val == "true"
+}
+
+func (c Container) getLabelValueOrEmpty(label string) string {
+	if val, ok := c.containerInfo.Config.Labels[label]; ok {
+		return val
+	}
+	return ""
+}
+
+func (c Container) getLabelValue(label string) (string, bool) {
+	val, ok := c.containerInfo.Config.Labels[label]
+	return val, ok
+}