Load foreign layers last

[bpcreech]

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?

As noted in #2099, if you use container_import to declare foreign layers, then depend on that in a container_image or container_push as a whole-image .tar, you lose the foreign layer URLs, resulting in an invalid image. This is because compat/reader.go loads the foreign image information from the image manifest first, then overwrites that with the empty stub layer found in the tarball. You can observe the loss of information in the resulting image's manifest.

This used to work with the Python-language image reader, but broke in the move to the Golang pusher.

Issue Number: #2099

What is the new behavior?

We load the foreign layers last so that the stub information is overwritten.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[gravypod]

LGTM.