Commit changes Dockerfile so that it creates a cleaner and lighter
final image using multi-stage approach.

Also switches to a debian bulsseye image. Build image using the normal
version while final image is based on slim version.

This commit adds 3 main features:
* Use single script entrypoint
* Provide routing to local network
* Block ALL traffic going outside of the VPN tunnel

Single script entrypoint allows one to avoid having to add service
features in the image.

Routing traffic can be configured by enduser using LOCAL_NETWORK
variable. And traffic drop is handled by iptables rules and policies.

Finally this commit adds a health check feature killing the container
as soon as VPN becomes unavailable.

Use gh-actions docker build and push to build multi-platform docker
image on tag push.

Supported platform are amd64, arm/v7 and arm64/v8

Fix container name typo

sysctl command ran by wg-quick when initiating tunnel is requiring
--privileged container which is unsecure.

Instead one can/should use docker --sysctl parameter.

This commit adds a condition in wg-quick script so that sysctl command
only runs when setting is not set to correct value