chore(xenial): fix container structure tests

[JGAntunes]

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Summary

Same as https://github.com/netlify/build-image/pull/867. Fixes the latest issues we've been experiencing on CI for our xenial release:

• https://app.circleci.com/pipelines/github/netlify/build-image/902/workflows/1fd9aaaa-6f83-48c7-acaa-a028d3b0d0da/jobs/2586/tests
• https://github.com/GoogleContainerTools/container-structure-test#metadata-test

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