[Request] Add HEALTHCHECK to the Dockerfile

[neoKushan]

I know the image itself contains a watch-guard script that runs in loop, but this is largely "invisible" to the Docker host (Beyond restarting the container).

If we were to add a HEALTHCHECK to the Dockerfile, possibly one that runs the watchguard script itself, it would give visibility to the host.

This would be particularly useful if you're using Privoxy in other containers, as we can specify dependencies on this image and Docker won't try to start them until the VPN connection is established and the container is "Healthy".

I'm sort-of able to do this myself by adding a healthcheck in my docker-compose file, but having it baked in would be quite useful for others.

If people like the idea, I don't mind taking a stab at it myself and doing a PR, but I didn't want to start if it wasn't welcomed/warranted.