Fix for Dockerfile smell DL3020

[grosa1]

Description

Hi!
The Dockerfile placed at "tests/randomized/docker/buster.Dockerfile" contains the best practice violation DL3020 detected by the hadolint tool.

The smell DL3020 occurs if ADD is used to copy files and directories that do not require tar self-extraction. In such cases, it is recommended to use the COPY instruction.
In this pull request, we propose a fix for that smell generated by our fixing tool. We have verified that the patch is correct before opening the pull request.
To fix this smell, specifically, the ADD instruction is replaced by an equivalent COPY instruction.

This change is only aimed at fixing that specific smell. If the fix is not valid or useful, please briefly indicate the reason and suggestions for possible improvements.

Thanks in advance.

Readiness checklist

• (only for Members) Changelog has been added to the release document.
• Tests added for this feature/bug.

Reviewer checklist

• Appropriate labels assigned.
• Milestone is set.
• Changelog has been added to the release document. For community contributors, the reviewer is in charge of this task.