add docker description

[bensteUEM]

The following is shown for "about this version" within the package section
https://github.com/bensteUEM/ChurchWebHelper/pkgs/container/church_web_helper/137579076?tag=latest

To provide a description, add the following line to your Dockerfile:
LABEL org.opencontainers.image.description DESCRIPTION

For multi-arch images, set a value for the org.opencontainers.image.description key in the annotations field of the manifest:
"annotations": { "org.opencontainers.image.description": "DESCRIPTION" }
Learn more about labelling container images