Merge bitcoin/bitcoin#25900: ci: run docker wrapper with a non-root user

849f20a6d3e437631a07469d3c4af5faa0aa06ed ci: create and use non-root user for docker image (josibake)

Pull request description:

  Previously, everything in the ci docker image ran as the root user. This would lead to certain directories (`ci/scratch`, `depends`) being owned by `root` after running the ci locally which would lead to annoying behavior such as subsequent guix builds failing due to `depends/` being owned by root.

  This PR adds a non-root user in the container and chowns the mounted working directory. All the `docker exec` commands now run as the non-root user, except for the few that still need to run as root (mainly, installing packages).

  To test this I checked out a fresh copy of the repo, applied my changes, ran the CI, and verified all the local file permissions were unchanged after the CI was finished running.

ACKs for top commit:
  hebasto:
    ACK 849f20a6d3e437631a07469d3c4af5faa0aa06ed, tested on Ubuntu 22.04 by running commands as follows:

Tree-SHA512: 734dca0f36157fce5fab243b4ff657fc17ba980e8e4e4644305f41002ff21bd5cef02c306ea1e0b5c841d4c07c095e8e4be16722e6a38c890717c60a3f5ec62a

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