Reduce docker image size (#394)

Reduce docker image size based on:  https://cadu.dev/reduce-your-docker-images-an-example-with-ruby/

docker/entrypoints/webpack.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 set -e
 
 rm -rf /app/tmp/pids/server.pid
@@ -17,4 +17,4 @@ done
 
 echo "Ready to run webpack development server."
 
-exec "$@"
+exec "$@"