gosu doesn't publish proper Go module semver tags, so `go install github.com/tianon/gosu@v1.19` fails with "no matching versions". Replace the multi-stage golang builder with `COPY --from=tianon/gosu /gosu /usr/local/bin/gosu`, which pulls the pre-built binary from the official tianon/gosu Docker image. This image is rebuilt with recent Go toolchains, so it still addresses the Go stdlib CVEs documented in the Dockerfile comments. Fixes #363 Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>