Tag the locally built LTS 2021 Docker image with 'latest-lts'

When building the Docker image locally with Maven, the image is tagged latest like the one for the 11.x version (on nuxeo/nuxeo).
It's hard to work on both repositories as we can't have both images built and available to Docker.

We want to tag the LTS 2021 one with a specific tag such as latest-lts.