CI: auto-generate list of published Docker image references

After building and uploading new Docker images for Autobahn, it would be great to have the new Docker images listed in a way that allows exact reference and direct use in other projects

Docker CLI and docker-compose allows to read env vars from a .env file, that if present, will make env vars already available during "docker-compose" phase (not only inside containers)

# Docker image references for Autobahn v21.1.1 (20220128-dc626948)
# https://hub.docker.com/r/crossbario/autobahn-python/tags?page=1&name=22.1.1

DOCKER_IMAGE_AUTOBAHN_PYTHON=crossbario/autobahn-python@sha256:dfed71b8441e219f2746ff08398d1571e99c666bf85bb4136fe2f9bf1a568650

Above can be computed like this

echo ${AUTOBAHN_VERSION}
echo ${AUTOBAHN_BUILD_ID}

docker inspect --format='{{index .RepoDigests 0}}' \
    crossbario/autobahn-python:cpy-amd64-${AUTOBAHN_VERSION}-${AUTOBAHN_BUILD_ID}

Start by locating the CI step that builds and uploads Autobahn's Docker images, then inspect how AUTOBAHN_VERSION and AUTOBAHN_BUILD_ID are exposed. Generate the requested .env-style output with image references resolved from RepoDigests, including the version, build ID, tag link, and exact digest; verify it can be consumed during docker-compose processing.