Convert all Dockerfile templates to multi-stage builds to eliminate
expensive `chown -R` operations. Use `COPY --from=build --chown=` to
set ownership in a single layer instead. Add venv to pip template for
clean multi-stage copying. Add coding agent files and test patterns to
dockerignore templates. Matches livekit/web#3448.
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
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