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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[jssmith]

Review — concise

Verdict: Approve. Simple, correct CI fix.

Summary

Replaces hardcoded crystaldba/postgres-mcp Docker Hub namespace with ${{ secrets.DOCKERHUB_USERNAME }}/postgres-mcp in the Docker build workflow. Applied to image tags and build cache references.

Correctness

• All four references updated consistently: images, cache-from, cache-to, and the metadata action.
• Using a secret for the namespace is the right approach — avoids hardcoding org names in CI config.
• No logic changes, just variable substitution.

Minor (non-blocking)

• Verify DOCKERHUB_USERNAME secret is set in the repo settings before merging, otherwise the build will push to /postgres-mcp (empty namespace).

This review was created by an AI agent (OpenHands) on behalf of @jssmith.