feat: add --health-check command line switch

cmd/root.go

@@ -152,6 +152,15 @@ func Run(c *cobra.Command, names []string) {
 	enableMetricsAPI, _ := c.PersistentFlags().GetBool("http-api-metrics")
 	unblockHTTPAPI, _ := c.PersistentFlags().GetBool("http-api-periodic-polls")
 	apiToken, _ := c.PersistentFlags().GetString("http-api-token")
+	healthCheck, _ := c.PersistentFlags().GetBool("health-check")
+
+	if healthCheck {
+		// health check should not have pid 1
+		if os.Getpid() == 1 {
+			os.Exit(1)
+		}
+		os.Exit(0)
+	}
 
 	if rollingRestart && monitorOnly {
 		log.Fatal("Rolling restarts is not compatible with the global monitor only flag")

dockerfiles/Dockerfile

@@ -17,4 +17,7 @@ COPY --from=alpine \
 EXPOSE 8080
 
 COPY watchtower /
+
+HEALTHCHECK CMD [ "/watchtower", "--health-check"]
+
 ENTRYPOINT ["/watchtower"]

dockerfiles/Dockerfile.dev-self-contained

@@ -35,4 +35,6 @@ COPY --from=builder /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/ca-certifi
 COPY --from=builder /usr/share/zoneinfo /usr/share/zoneinfo
 COPY --from=builder /watchtower/watchtower /watchtower
 
+HEALTHCHECK CMD [ "/watchtower", "--health-check"]
+
 ENTRYPOINT ["/watchtower"]

dockerfiles/Dockerfile.self-contained

@@ -35,4 +35,6 @@ COPY --from=builder /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/ca-certifi
 COPY --from=builder /usr/share/zoneinfo /usr/share/zoneinfo
 COPY --from=builder /go/watchtower/watchtower /watchtower
 
+HEALTHCHECK CMD [ "/watchtower", "--health-check"]
+
 ENTRYPOINT ["/watchtower"]

docs/arguments.md

@@ -363,4 +363,15 @@ requests and may rate limit pull requests (mainly docker.io).
 Environment Variable: WATCHTOWER_WARN_ON_HEAD_FAILURE
      Possible values: always, auto, never
              Default: auto
-```
+```
+
+## Health check
+
+Run health check and exit.
+
+```text
+            Argument: --health-check
+Environment Variable: WATCHTOWER_HEALTH_CHECK
+     Possible values: -
+             Default: -
+```

internal/flags/flags.go

@@ -186,6 +186,12 @@ func RegisterSystemFlags(rootCmd *cobra.Command) {
 		"log-level",
 		viper.GetString("WATCHTOWER_LOG_LEVEL"),
 		"The maximum log level that will be written to STDERR. Possible values: panic, fatal, error, warn, info, debug or trace")
+
+	flags.BoolP(
+		"health-check",
+		"",
+		viper.IsSet("WATCHTOWER_HEALTH_CHECK"),
+		"Do health check and exit")
 }
 
 // RegisterNotificationFlags that are used by watchtower to send notifications