Add CI workflow for automated container image release to GitHub Container Registry

[Copilot]

Overview

This PR adds a GitHub Actions CI workflow that automatically builds and releases container images to GitHub Container Registry (ghcr.io) as requested in the issue.

Changes

GitHub Actions CI Workflow (.github/workflows/ci.yml)

Created a comprehensive CI/CD pipeline with two jobs:

1. Build and Test Job

• Runs on all pushes and pull requests
• Sets up Go 1.24 environment
• Downloads dependencies and builds the application
• Executes tests to ensure code quality

2. Build and Push Image Job

• Runs only on push events (skips pull requests)
• Authenticates with GitHub Container Registry using GITHUB_TOKEN
• Builds Docker image using the existing Dockerfile
• Automatically tags and pushes images with intelligent tagging strategy:
  • Branch-based tags (e.g., main, main-<sha>)
  • Semantic version tags for releases (e.g., v1.0.0, 1.0, 1)

Documentation Updates

Updated README.md with a new section explaining:

• How the automated CI/CD process works
• Trigger conditions for image releases
• Image tagging rules
• Usage examples for pulling released images

Other Improvements

• Updated .gitignore to exclude build artifacts (api binary)

Usage

After this PR is merged, container images will be automatically published to ghcr.io on every push to main or when version tags are created:

# Pull latest main version
docker pull ghcr.io/gthulhu/api:main

# Pull specific version
docker pull ghcr.io/gthulhu/api:v1.0.0

Testing

• Verified Go application builds successfully
• Validated GitHub Actions workflow syntax
• Confirmed .gitignore excludes build artifacts

Fixes the issue by implementing automated container image release in the CI workflow.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Release container image on GitHub package</issue_title>
<issue_description>As title, add image release step in ci workflow.</issue_description>

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

Fixes #2

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