add downloader script for JVS's ffmpeg master blds

ffmpeg-from-master.sh

@@ -0,0 +1,15 @@
+#!/bin/sh
+case $(uname -m) in
+  x86_64)
+    ARCH=amd64;;
+  aarch64)
+    ARCH=arm64;;
+  armv7)
+    ARCH=armel;;
+  armv7l)
+    ARCH=armel;;
+  *)
+    ARCH=$(uname -m);;
+esac
+wget "https://johnvansickle.com/ffmpeg/builds/ffmpeg-git-$ARCH-static.tar.xz" -O ffmpeg.txz
+docker build