The docker images are build locally on the developer machine:
cd .circleci/docker/
docker build -t ethereum/solc-buildpack-deps:ubuntu1904 -f Dockerfile.ubuntu1904 .
docker push solidity/solc-buildpack-deps:ubuntu1904
which you can find on Dockerhub after the push at:
https://hub.docker.com/r/ethereum/solidity-buildpack-deps
where the image tag reflects the target OS to build Solidity and run its test on.