fix: ensure /tmp is writable by container user in Docker image

The scratch-based Docker image copies /tmp from the builder stage with
root ownership, but the container runs as UID 1000. SQLite needs a
writable temp directory for complex transactions (task position changes,
saved filter creation, data export), causing "disk I/O error: permission
denied" errors.

Closes go-vikunja/vikunja#2316

Dockerfile

@@ -50,7 +50,7 @@ WORKDIR /app/vikunja
 ENTRYPOINT [ "/app/vikunja/vikunja" ]
 EXPOSE 3456
 
-COPY --from=apibuilder /tmp /tmp
+COPY --from=apibuilder --chown=1000:1000 /tmp /tmp
 
 USER 1000