fix(docker): copy go.sum so the server image builds

[qiansheng91]

Problem

deployments/docker/Dockerfile copies only go.mod into the build stage:

COPY go.mod ./

go.sum is therefore absent from the build context, and go build fails:

internal/umodel/schemaspec/registry.go:11:2: missing go.sum entry for module providing package gopkg.in/yaml.v3 (imported by github.com/alibaba/UnifiedModel/internal/umodel); to add:
	go get github.com/alibaba/UnifiedModel/internal/umodel

Fix

Copy go.sum alongside go.mod and pre-download modules before building — the standard multi-stage Go pattern:

COPY go.mod go.sum ./
RUN go mod download

go.sum already exists at the repo root, so it just needs to be added to the COPY.

Verification

Built with the build context = repo root (docker build -f deployments/docker/Dockerfile .):

• Before (original Dockerfile): fails at go build with the missing go.sum entry error above.
• After (this change): image builds successfully through both stages.