Update externals for new gn and add helper script (#1591)

externals/skia

@@ -1 +1 @@
-Subproject commit 36131398f04bfa8dcb79c2f64ecf2c80d2ef565f
+Subproject commit d8e22c4421640690c7bbedf865663b0dba6c7e2d

scripts/Docker/alpine/amd64/.dockerignore

@@ -0,0 +1 @@
+build-local.sh

scripts/Docker/alpine/amd64/build-local.sh

@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+set -e
+
+DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null 2>&1 && pwd )"
+
+(cd $DIR && docker build --tag skiasharp-alpine .)
+(cd $DIR/../../../../ && docker run --rm --name skiasharp-alpine --volume $(pwd):/work skiasharp-alpine /bin/bash ./bootstrapper.sh -t externals-linux -c Release --buildarch=x64 --variant=alpine)