Add GitHub Actions workflow for Docker image publishing to GHCR

[Copilot]

Automates Docker image builds and publishes to ghcr.io/hackucf/publix-discount-detector using the existing Dockerfile.k8s.

Workflow Configuration

• Triggers: Push to main, version tags (v*.*.*), PRs (build-only), manual dispatch
• Authentication: Built-in GITHUB_TOKEN with packages: write permission
• Tagging: Semantic versions, SHA commits, branch refs, and latest for main branch
• Build optimization: GitHub Actions cache, Docker Buildx

Example Usage

After merge, the image will be available at:

docker pull ghcr.io/hackucf/publix-discount-detector:latest
docker pull ghcr.io/hackucf/publix-discount-detector:v1.0.0
docker pull ghcr.io/hackucf/publix-discount-detector:sha-abc1234

Version tags create multiple image tags automatically (v1.2.3 → v1.2.3, v1.2, v1).

Original prompt

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This section details on the original issue you should resolve

<issue_title>Create a github workflows pipeline to push docker image to ghcr.io</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Create a github workflows pipeline to push docker image to ghcr.io #1

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