Release v2.1.1

This release fixes several issues (#37 #38 #39) and implements a new feature -- a Helm chart (#34).

The GitHub workflow has been updated to automatically create helm charts (which correspond to release version) and create a release when tagging.

The server can now be simply installed into Kubernetes by issuing the following commands:

helm repo add bokysan https://bokysan.github.io/docker-postfix/
helm upgrade --install --set persistence.enabled=false --set config.general.ALLOWED_SENDER_DOMAINS=example.com mail bokysan/mail