Add org.opencontainers.image.source label (#193)

Two image-label fixes:

- Add `org.opencontainers.image.source` so tools like renovate can retrieve release notes from the source repo.
- Re-declare `ARG VERSION` in the final stage. `ARG` after `FROM` is stage-scoped, so the builder-stage `VERSION` never reached the final stage and `org.opencontainers.image.version` expanded to an empty string in published images.

Closes #192

---
This PR was written with the help of Claude Opus 4.7

Reviewed-on: https://gitea.com/gitea/gitea-mcp/pulls/193
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-committed-by: silverwind <me@silverwind.io>

Dockerfile

@@ -22,11 +22,14 @@ RUN --mount=type=cache,target=/go/pkg/mod \
 # Final stage
 FROM gcr.io/distroless/static-debian12:nonroot
 
+ARG VERSION=dev
+
 WORKDIR /app
 COPY --from=builder --chown=nonroot:nonroot /app/gitea-mcp .
 
 USER nonroot:nonroot
 
 LABEL org.opencontainers.image.version="${VERSION}"
+LABEL org.opencontainers.image.source="https://gitea.com/gitea/gitea-mcp"
 
 CMD ["/app/gitea-mcp"]