fix(docker): align Go toolchain and validate multi-platform images

[Zxilly]

Summary

• Align the Docker builder with the Go 1.24.1 toolchain already declared in go.mod.
• Build linux/amd64 and linux/arm64/v8 images in pull request CI without publishing or registry credentials.

Why

The post-merge image build for #266 fails because Go 1.22.12 cannot build the module, which now requires Go >= 1.23.0.
Failure: https://github.com/ZNotify/server/actions/runs/37629925018/job/112822742458

Validation

• Workflow YAML parsed and architecture/no-publish assertions passed.
• git diff --check passed.
• All 30 PR CI jobs passed on bafaae6: lint, server unit tests, three-platform binaries, 24 SDK integration jobs, and the amd64/arm64 Docker build.
• CI: https://github.com/ZNotify/server/actions/runs/37737456830
• Codacy and the additional summary check passed; Mergify reported neutral. No failing or pending checks remain.

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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[mergify]

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