Fix entrypoint shell

Bash is not provided by default on alpine linux. The script is not using 
any bash specific features, which meakes it easy to replace it with ash.

entrypoint.sh

-#!/bin/bash
+#!/bin/sh
 
 if [ "$UPSTREAM_NAME" != "" ]; then
     UPSTREAM="$(getent hosts "$UPSTREAM_NAME" | awk '{print $1}'):53"