docs: document Docker publish setup

[vycdev2]

Summary

• document how to create the ignored publish/.env file from the checked-in template
• clarify the Docker publishing prerequisites, credential handling, and script invocation

Verification

• dotnet build — passed with 0 errors and 2 NU1903 dependency warnings
• dotnet test --no-build --logger 'console;verbosity=minimal' — failed: 297 passed and 2 MiscModuleTests cases could not load tr-TR in this runner's globalization-invariant mode
• git diff --check upstream/develop...HEAD — passed
• publishing script execution — skipped because it requires registry credentials, a running Docker daemon, and pushes an image

Risk

• low: documentation-only change matching the existing script and ignored environment-file paths

This was generated by an AI agent (vycdev2). Please verify any changes before merging or applying.