feat: add --health-check command line switch

cmd/root.go

@@ -140,6 +140,16 @@ 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 {
+			time.Sleep(1 * time.Second)
+			log.Fatal("The health check flag should never be passed to the main watchtower container process")
+		}
+		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

@@ -419,6 +419,17 @@ Environment Variable: WATCHTOWER_WARN_ON_HEAD_FAILURE
              Default: auto
 ```
 
+## Health check
+
+Returns a success exit code to enable usage with docker `HEALTHCHECK`. This check is naive and only returns checks whether there is another process running inside the container, as it is the only known form of failure state for watchtowers container.
+
+!!! note "Only for HEALTHCHECK use"
+    Never put this on the main container executable command line as it is only meant to be run from docker HEALTHCHECK.
+
+```text
+            Argument: --health-check
+```
+
 ## Programatic Output (porcelain)
 
 Writes the session results to STDOUT using a stable, machine-readable format (indicated by the argument VERSION).  

internal/flags/flags.go

@@ -193,9 +193,15 @@ func RegisterSystemFlags(rootCmd *cobra.Command) {
 		"The maximum log level that will be written to STDERR. Possible values: panic, fatal, error, warn, info, debug or trace")
 
 	flags.BoolP(
+		"health-check",
+		"",
+		false,
+		"Do health check and exit")
+
+  flags.BoolP(
 		"label-take-precedence",
 		"",
-		viper.GetBool("WATCHTOWER_LABEL_TAKE_PRECEDENCE"),
+		envBool("WATCHTOWER_LABEL_TAKE_PRECEDENCE"),
 		"Label applied to containers take precedence over arguments")
 }