Add empty go.sum file (no external dependencies yet) and update the
Dockerfile build stage to COPY go.sum alongside go.mod for reproducible
module downloads. This ensures the Docker build does not fail when Go
requires go.sum to be present.
Closesleeworks-agents/gitea-mobile#203Closesleeworks-agents/gitea-mobile#180
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>