Add GitHub Actions workflow for multi-arch API Docker builds

• Understand current resolve-tag logic (manual git describe + patch increment)
• Add GitVersion.yml config file (ContinuousDeployment mode, Patch increment on main)
• Replace resolve-tag job to use gittools/actions (setup + execute) instead of manual shell script
• Remove tag-push special case (GitVersion handles it natively)
• Validate YAML syntax and check for advisories