Docker: Allow to use image like kas command

This adds a bit heuristic to the docker entrypoint in order to move our
API towards "docker run [...] kasproject/kas build kas.yml" - without
breaking existing users. And now you can also do

    docker run --rm kasproject/kas --version

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

docker-entrypoint

@@ -29,7 +29,14 @@ fi
 
 cd /builder
 if [ -n "$1" ]; then
-	exec $GOSU "$@"
+	case "$1" in
+	build|shell|-*)
+		exec $GOSU kas "$@"
+		;;
+	*)
+		exec $GOSU "$@"
+		;;
+	esac
 else
 	exec $GOSU bash
 fi