Overwrite existing dest when copying symlink and preserve link target

[aduong]

This behaviour is in line with the Docker builder reference:

If <dest> does not end with a trailing slash, it will be considered a regular file and the contents of <src> will be written at <dest>.

— https://docs.docker.com/engine/reference/builder/#copy

Fixes #442, #437

[container-tools-bot]

Hi @aduong. Thanks for your PR.

I'm waiting for a GoogleContainerTools member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

[aduong]

I see that the integration test run was unsuccessful. It succeeds locally but I don't have access to the kokoro data. How should I proceed here?

[priyawadhwa]

@aduong looks like a flaky test, I'll rerun kokoro.

[aduong]

Looks like that was it. All checks pass now.

[priyawadhwa]

LGTM