contrib : add ngxson as codeowner for server, devops

[ngxson]

I didn't notice that CODEOWNERS is now available in the repo.

Therefore, I would like to apply to be maintainer for devops (docker images) and server part.

I also changed ci/ to /ci/ in the file, because according to github docs:

# The `docs/*` pattern will match files like
# `docs/getting-started.md` but not further nested files like
# `docs/build-app/troubleshooting.md`.
docs/* docs@example.com

# In this example, @octocat owns any file in an apps directory
# anywhere in your repository.
apps/ @octocat

# In this example, @doctocat owns any file in the `/docs`
# directory in the root of your repository and any of its
# subdirectories.
/docs/ @doctocat