Make the skia-wasm-release image only have the build products

The previous version had several gigs of buildtools, source code, etc.
This is much smaller and easier to download for tools like
jsfiddles.skia.org and (soon) skottie.skia.org

Bug: skia:
Change-Id: I748487abb1c6f6e82476076d22a27a5d4f90f75a
Reviewed-on: https://skia-review.googlesource.com/c/169240
Reviewed-by: Kevin Lubick <kjlubick@google.com>
diff --git a/docker/skia-wasm-release/Dockerfile b/docker/skia-wasm-release/Dockerfile
index 9520979..49cc27e 100644
--- a/docker/skia-wasm-release/Dockerfile
+++ b/docker/skia-wasm-release/Dockerfile
@@ -1,5 +1,5 @@
 # Dockerfile for building the WASM libraries used by jsfiddle.skia.org
-FROM gcr.io/skia-public/emsdk-release:prod
+FROM gcr.io/skia-public/emsdk-release:prod as builder
 
 RUN cd /tmp \
   && git clone 'https://chromium.googlesource.com/chromium/tools/depot_tools.git' \
@@ -16,3 +16,23 @@
 
 # CanvasKit should be in /tmp/skia/skia/out/canvaskit_wasm
 RUN /tmp/skia/skia/experimental/canvaskit/compile.sh
+
+RUN cd /tmp/skia/skia && git rev-parse HEAD > /tmp/VERSION
+
+#############################################################################
+# Multi-stage build part 2, in which we only have the compiled results and
+# a VERSION in /tmp
+# See https://docs.docker.com/develop/develop-images/multistage-build/
+#############################################################################
+
+FROM alpine:latest
+
+WORKDIR /tmp/
+
+RUN mkdir /tmp/pathkit /tmp/canvaskit
+
+COPY --from=builder /tmp/VERSION /tmp/VERSION
+
+COPY --from=builder /tmp/skia/skia/out/pathkit/pathkit* /tmp/pathkit/
+
+COPY --from=builder /tmp/skia/skia/out/canvaskit_wasm/canvaskit* /tmp/canvaskit/