Cache loaded images for performance improvements.

[aptenodytes-forsteri]

Locally on ubuntu 18.04, the join_layers step takes

30 seconds for the container_bundle_with_install_pkgs
target without this change, and ~5 seconds with this change.

With the previous implementation, join_layers was passing the same
set of images to reader.parts for each call to ReadImage. The reader,
created fresh for each call to ReadImage, would then load these same
images again.

This results in a fixed cost for every image you add to the bundle. For
large bundles, this can be very costly (on the order of minutes).

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

Other information

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[alexeagle]

Thanks for the performance fix! Can you sign the CLA?

[alexeagle]

CI compile failure
docker/container/go/cmd/digester/digester.go:52:14: undefined: compat.ReadImage

Also it wants a lint fix

[aptenodytes-forsteri]

@googlebot I signed it!

[aptenodytes-forsteri]

Seems like too much of a coincidence for this to timeout on my branch (all recent builds seem to be successful). However, I can't seem to reproduce this locally. I figure I probably broke something with pushing or pulling docker images?

(04:21:27) ERROR: An error occurred during the fetch of repository 'alpine_linux_armv6':
  | Traceback (most recent call last):
  | File "/workdir/container/pull.bzl", line 216, column 13, in _impl
  | fail("Pull command failed: %s (%s)" % (result.stderr, " ".join([str(a) for a in args])))
  | Error in fail: Pull command failed: Timed out (/var/lib/buildkite-agent/.cache/bazel/_bazel_buildkite-agent/ec321eb2cc2d0f8f91b676b6d4c66c29/external/go_puller_linux_amd64/file/downloaded -directory /var/lib/buildkite-agent/.cache/bazel/_bazel_buildkite-agent/ec321eb2cc2d0f8f91b676b6d4c66c29/external/alpine_linux_armv6/image -os linux -os-version  -os-features  -architecture arm -variant v6 -features  -name index.docker.io/library/alpine@sha256:dabea2944dcc2b86482b4f0b0fb62da80e0673e900c46c0e03b45919881a5d84)
 ```

[gravypod]

LGTM. Other performance improvements are welcome. I'd love it if we had someone look at converting build_tar.py to golang.