Add build script

Author: rpidanny

build.sh

@@ -0,0 +1,6 @@
+#!/bin/sh
+
+export FFMPEG_VERSION=4.1.3
+
+docker build --build-arg FFMPEG_VERSION -t ffmpeg-lambda-layer .
+docker run --rm ffmpeg-lambda-layer cat /tmp/ffmpeg-${FFMPEG_VERSION}.zip > ./layer.zip