Add CI workflow to publish Docker images to ghcr.io/init4tech

[init4samwise]

This PR adds a GitHub Actions workflow that:

• Triggers on tagged releases (v*)
• Builds Docker images using the existing docker/Dockerfile
• Pushes to ghcr.io/init4tech/blockscout
• Tags images with the git tag version (e.g., 1.2.3), major.minor (e.g., 1.2), and latest
• Supports linux/amd64 and linux/arm64 platforms
• Uses GITHUB_TOKEN for ghcr.io authentication (built-in)
• Uses GitHub Actions cache for faster builds

To publish a new image, simply create and push a tag like v1.0.0.

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Related to init4tech's Blockscout deployment infrastructure.