change base image to alpine tag

Dockerfile

-FROM nginx:1.9
+FROM nginx:alpine
 
 ADD run.sh /run.sh
 ADD default.conf /etc/nginx/conf.d/default.conf

run.sh

-#!/usr/bin/env bash
+#!/usr/bin/env sh
 
 if [ ! -n "$SERVER_REDIRECT" ] ; then
     echo "Environment variable SERVER_REDIRECT is not set, exiting."