fix: resolve Lambda invocation failure from API Gateway

- Align Dockerfile with AWS official documentation for provided runtime
- Remove DefinitionBody from SAM template to avoid conflict with Events property
- This enables SAM to automatically generate AWS::Lambda::Permission resource
- Fixes 500 error when Forgejo webhook triggers API Gateway endpoint"

docker/Dockerfile

@@ -13,6 +13,5 @@ FROM public.ecr.aws/lambda/provided:al2023
 RUN dnf update -y && \
     dnf install -y git zip && \
     dnf clean all
-COPY --from=build /app/main ${LAMBDA_TASK_ROOT}
-WORKDIR ${LAMBDA_TASK_ROOT}
+COPY --from=build /app/main ./main
 ENTRYPOINT [ "./main" ]