Add OCI Container support with automated GitHub Actions publishing

[aaronjbrown]

Do you love how frequently fiddyschmitt is updating condeco-cli but hate having to manually download/update the software like it's the 1990's?!

This PR adds the ability to run condeco-cli as a Docker container, with automated image builds via GitHub Actions.

Changes

• Dockerfile – Packages the Linux x64 binary into a minimal debian:bookworm-slim image. Includes libicu72 and ca-certificates as the only runtime dependencies.
• .github/workflows/docker-publish.yml – GitHub Actions workflow that automatically builds and publishes the container image to GitHub Container Registry (GHCR) whenever a new release is published. Can also be triggered manually against any existing release tag.
• README.md – Added a Docker section documenting how to pull the image and run it with a mounted config file, including a note for SELinux-enabled systems.

Usage

docker run --rm \
  -v /path/to/condeco-cli.json:/app/condeco-cli.json \
  ghcr.io/aaronjbrown/condeco-cli:latest --checkin

Scheduling is handled externally by calling docker run from the host cron or Scheduled Task, consistent with the existing scheduling approach documented in the wiki.