[WIP] Refactor Dockerfile

[Raimo33]

This is a draft PR to refactor the dockerfile according to issues:

• Refactor Dockerfile after the removal of all built-in Python plugins #8409
• Docker: verify GPG keys #8405

[ShahanaFarooqui]

@Raimo33 As per @cdecker's suggestion here, we should modify the Dockerfile to run as a non-root user for improved security. Since we're already refactoring the Dockerfile, could we incorporate this change as well?

[Raimo33]

I'm removing the manual handling of tini in favor of a user controlled approach, more docker native and friendly.

ENTRYPOINT  [ "/usr/bin/tini", "-g", "--", "./entrypoint.sh" ]

becomes:

ENTRYPOINT  ["/entrypoint.sh"]

and the user is free to run the container with tini by using the official --init flag as follows:

docker run --init ...