ci: add GHCR release workflow and multi-arch image publishing

[Izzetee]

Summary

Adds Docker release automation so tagged versions publish a ready-to-run container image.

Changes

• New workflow .github/workflows/release.yml:
  • Triggers on v*.*.* tag pushes and via manual workflow_dispatch (with a tag input, so the existing v0.6.0 tag can be built retroactively).
  • Builds a multi-arch image (linux/amd64, linux/arm64) with QEMU + Buildx.
  • Publishes to GHCR at ghcr.io/izzetee/pipimink with vX.Y.Z, X.Y, and latest tags.
  • Creates a GitHub Release, extracting notes from the matching CHANGELOG.md section.
• Dockerfile: made the providers.json COPY resilient. providers.json and .env are gitignored, so a fresh CI checkout does not contain them; listing the always-tracked providers.example.json keeps the COPY from failing while still copying the real file for local builds.
• README.md: documents the published GHCR image and pull command.
• CHANGELOG.md: [Unreleased] note for the release automation.

Notes

• Uses only GITHUB_TOKEN (no extra secrets); packages: write permission is scoped to this workflow.
• The image name is lowercased by docker/metadata-action, so Izzetee/PiPiMink maps to ghcr.io/izzetee/pipimink.
• After merge, the v0.6.0 tag already exists and will not retro-trigger on push, so the first image can be published by running the workflow manually with input v0.6.0.