Sign Docker images

[docktermj]

Currently, the docker images that are uploaded to dockerhub are not signed, which means for a user who is pulling the image, there is no way to verify whether the image was actually built and pushed from github. This is part of a larger effort to improve senzing's supply chain security, and more specifically to prevent an attacker who has stolen access to the senzing's dockerhub account and starts pushing malicious images up to the cloud.

GitHub now offers a new service to sign containers. This request is to sign the Senzing-created containers.

References:

1. https://github.blog/2021-12-06-safeguard-container-signing-capability-actions/
2. https://github.com/sigstore/cosign#registry-support