container: Drop privileges

A container is best served with the least amount of privileges. This
also ensures we don't have to drop anything later.

This does require running the container with elevated capabilities.

Note, that if for whatever reason, 'root' access within the container is
needed, this can easily be accomplished by running the container with
`docker run --user root:root sslh` for example.

Signed-off-by: Olliver Schinagl <oliver@schinagl.nl>

Dockerfile

@@ -22,3 +22,5 @@ RUN apk --no-cache add libconfig pcre2
 
 COPY "./container-entrypoint.sh" "/init"
 ENTRYPOINT [ "/init" ]
+
+USER nobody:nogroup

README.md

@@ -47,6 +47,8 @@ How to use
 
 ```bash
 docker run \
+  --cap-add CAP_NET_RAW \
+  --cap-add CAP_NET_BIND_SERVICES \
   --rm \
   -it \
   ghcr.io/yrutschle/sslh:latest \