feat: add --health-check command line switch (#1725)

Co-authored-by: nils måsén <nils@piksel.se>

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).