builder/docker: upload files with -a flag when upstream bug is fixed

[ghost]

This issue was originally opened by @mwhooker as hashicorp/packer#5409. It was migrated here as a result of the Packer plugin split. The original body of the issue is below.

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We should set ownership of files based on the container user. In theory, this should be doable with the -a flag to docker cp, but there's a bug that prevents that. When moby/moby#34142 is fixed, let's change docker cp to be docker cp -a