Rework cache key generation a bit.

[dlorenc]

Cache keys are now based on the previous commands, rather than the previous state
of the filesystem.

This will allow us to add some further optimizations to caching.

Still needs some tests.

[dlorenc]

/wip

[dlorenc]

This should be ready for a review now.

[priyawadhwa]

I really like the CompositeCache idea! So I was using the current state of the filesystem for Dockerfiles like this:

FROM gcr.io/google-appengine/debian9@sha256:1d6a9a6d106bd795098f60f4abb7083626354fa6735e81743c7f8cfca11259f0
RUN apt-get update && apt-get install -y make
COPY foo /foo

With this switch, if foo is changed then the entire Dockerfile will be rebuilt, even though the first RUN command may have been cached on a previous run and could still be used.

Instead of the entire source context, WDYT of adding the snapshotted files as they're added by each command? (I think that should work but not 100% sure).

[dlorenc]

Whoo, finally passing!

[priyawadhwa]

LGTM!