Docker installation broken

Fix dockerfile after switching to poetry and add deployment via CI/CD

Fixed in #498: the Dockerfile was rewritten as a two stage uv build, and .github/workflows/docker.yml builds, smoke tests and publishes the image to ghcr.io/liiight/notifiers (linux/amd64 and linux/arm64). :main is published from main now; latest and version tags come with the 2.0.0 release. See the Docker section of the installation docs.