Change cache key calculation to be more reproducible. (#525)

Before we were using the full image digest, but that contains a timestamp. Now
we only use the layers themselves and the image config (env vars, etc.).

Also fix a bug in unpacking the layers themselves. mtimes can change during unpacking,
so set them all once at the end.

integration/dockerfiles/Dockerfile_test_cache

@@ -16,7 +16,15 @@
 # If the image is built twice, /date should be the same in both images
 # if the cache is implemented correctly
 
-FROM gcr.io/google-appengine/debian9@sha256:1d6a9a6d106bd795098f60f4abb7083626354fa6735e81743c7f8cfca11259f0
+FROM alpine as base_stage
+
+RUN mkdir foo && echo base_stage > foo/base
+
+FROM base_stage as cached_stage
+
+RUN echo cached_stage > foo/cache
+
+FROM cached_stage as bug_stage
+
+RUN echo bug_stage > foo/bug
 RUN date > /date
-COPY context/foo /foo
-RUN echo hey