replace ADD with COPY in Dockerfile

[cjber]

PR Checklist

• A description of the changes is added to the description of this PR.
• If there is a related issue, make sure it is linked to this PR.
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

Description of changes

Changed the ADD instruction to COPY in the Dockerfile. This is recommended for copying normal files or directories into the container, and conforms with best practices.

https://www.docker.com/blog/docker-best-practices-understanding-the-differences-between-add-and-copy-instructions-in-dockerfiles/