fix: allow Docker builds without BUILD_VERSION

[jacob-sheng]

Summary

• fix the Docker go-builder stage so BUILD_VERSION is optional under set -u
• keep the existing version resolution order: explicit build arg, then repo VERSION, then empty value
• document the fallback behavior for Zeabur and regular Docker builds in both docs and the Zeabur template

Root cause

Zeabur builds the repo Dockerfile without passing BUILD_VERSION. The current shell step expands ${BUILD_VERSION} before fallback logic runs, so /bin/sh exits with parameter not set.

Testing

• verified shell fallback behavior with set -u when BUILD_VERSION is unset
• verified explicit BUILD_VERSION still wins when provided
• ran git diff --check
• could not run full docker build or Go tests in this environment because docker and go are unavailable

[vercel]

@jacob-ayang is attempting to deploy a commit to the cjack's projects Team on Vercel.

A member of the Team first needs to authorize it.

[jacob-sheng]

按作者要求关闭，稍后会以新的分支、中文描述和统一身份重新提交。