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" ]

container-entrypoint.sh

@@ -0,0 +1,24 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL2-or-later
+#
+# Copyright (C) 2023 Olliver Schinagl <oliver@schinagl.nl>
+#
+# A beginning user should be able to docker run image bash (or sh) without
+# needing to learn about --entrypoint
+# https://github.com/docker-library/official-images#consistency
+
+set -eu
+
+bin='sslh'
+
+# run command if it is not starting with a "-" and is an executable in PATH
+if [ "${#}" -le 0 ] || \
+   [ "${1#-}" != "${1}" ] || \
+   [ -d "${1}" ] || \
+   ! command -v "${1}" > '/dev/null' 2>&1; then
+	entrypoint='true'
+fi
+
+exec ${entrypoint:+${bin}} "${@}"
+
+exit 0