Add entrypoint script

entrypoint.sh

@@ -0,0 +1,21 @@
+#!/bin/sh
+
+# Sample from https://github.com/traefik/traefik-library-image/blob/5070edb25b03cca6802d75d5037576c840f73fdd/v3.1/alpine/entrypoint.sh
+
+set -e
+
+# first arg is `-f` or `--some-option`
+if [ "${1#-}" != "$1" ]; then
+    set -- gerbil "$@"
+fi
+
+# if our command is a valid Gerbil subcommand, let's invoke it through Gerbil instead
+# (this allows for "docker run gerbil version", etc)
+if gerbil "$1" --help >/dev/null 2>&1
+then
+    set -- gerbil "$@"
+else
+    echo "= '$1' is not a Gerbil command: assuming shell execution." 1>&2
+fi
+
+exec "$@"