Renovate only tracked the engine version in ci/dagger.json. Add regex
customManagers so it also updates DAGGER_VERSION in .forgejo/Dockerfile
and the nix flake reference in DAGGER.md, keeping all three in sync.
Also bumps the stale DAGGER.md nix reference from v0.11.4 to v0.20.8
to match the current engine version.
Closes#358
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>