docker: Add proper entrypoint

As per docker guidelines [0] a container should always really have a
consistent entrypoint, without having to override it or do special
tricks.

The behavior should be _identical_ as before, but will no longer trigger
errors because sslh doesn't understand certain parameters (/bin/sh
for example being common). Further more, allows a proper entrypoint for
a CI to work easily with the container as well. Allowing for scenario's
such as `apk add git && sslh --foreground` in your sslh image for example.

E.g. `docker run sslh --help` works though with the default
`--foreground` a bit weirdly, as does `docker run sslh
/bin/sh` or `docker run sslh ls`.

[0]: https://github.com/docker-library/official-images#consistency

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

Dockerfile

@@ -20,4 +20,5 @@ COPY --from=build "/sslh/sslh-select" "/usr/local/bin/sslh"
 
 RUN apk --no-cache add libconfig pcre2
 
-ENTRYPOINT [ "/usr/local/bin/sslh" ]
+COPY "./container-entrypoint.sh" "/init"
+ENTRYPOINT [ "/init" ]