fix(ci): use DOCKERHUB_USERNAME secret instead of hardcoded registry namespace

[EliShteinman]

Summary

• The DockerHub workflow had crystaldba/postgres-mcp hardcoded as the image name and cache registry references
• This caused Docker builds to fail on forks, since the forked repo's DockerHub credentials don't have push access to crystaldba/postgres-mcp
• Replaced all hardcoded crystaldba references with ${{ secrets.DOCKERHUB_USERNAME }} so each fork can configure its own DockerHub namespace

[Copilot]

Pull request overview

This PR updates the DockerHub build/push GitHub Actions workflow to make the Docker image namespace configurable via a secret, avoiding hardcoded crystaldba/... references that break builds on forks.

Changes:

• Replace hardcoded crystaldba/postgres-mcp image name with ${{ secrets.DOCKERHUB_USERNAME }}/postgres-mcp
• Update registry build cache ref values to use ${{ secrets.DOCKERHUB_USERNAME }} as well

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