More portable setup scripts (#22)

* update shebang
* busybox and ash and others don't use BASH_SOURCE, but this is all
  Golang so we can assume $GOPATH is set

Author: matthewpoer

scripts/build_binary.sh

@@ -1,12 +1,12 @@
-#!/bin/bash
+#!/usr/bin/env sh
 
 set -e
 
 # Copyright 2019 Amazon.com, Inc. or its affiliates. All Rights Reserved.
 # SPDX-License-Identifier: MIT-0
 
 # Normalize to working directory being build root (up one level from ./scripts)
-ROOT=$( cd "$( dirname "${BASH_SOURCE[0]}" )/.." && pwd )
+ROOT="$GOPATH/src/github.com/awslabs/aws-lambda-container-image-converter/"
 cd "${ROOT}"
 
 # Builds the binary from source in the specified destination paths.