feat(docker): add dockerfile for goreleaser

[RobBrazier]

Bundle the cli alongside supercronic - so you don't have to worry about setting up a cron job to trigger a docker container

fixes #4

docker run ghcr.io/robbrazier/readflow will run with an hourly cron job (overridable with CRON_SCHEDULE environment variable
docker run ghcr.io/robbrazier/readflow sync will trigger the sync and exit

[deepsource-io]

Here's the code health analysis summary for commits c154c82..472ae69. View details on DeepSource ↗.

Analysis Summary

Analyzer	Status	Summary	Link
Shell	✅ Success		View Check ↗
Go	✅ Success		View Check ↗

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