buildx failed with: ERROR: failed to solve: process "/bin/sh -c DEBIAN_FRONTEND=noninteractive apt update && apt upgrade -yq && apt install -yq --install-recommends sudo software-properties-common build-essential ca-certificates cmake cmake-curses-gui file gcc g++ gfortran git libblas-dev liblapack-dev libopenmpi-dev lsb-release ninja-build numdiff openmpi-bin openmpi-common wget zlib1g-dev libboost-all-dev libnetcdf-dev && add-apt-repository $REPO && apt update && apt install -yq libdeal.ii-$VERSION libdeal.ii-dev && apt-get clean && rm -r /var/lib/apt/lists/*" did not complete successfully: exit code: 100

The hosted runner: GitHub Actions 6 lost communication with the server. Anything in your workflow that terminates the runner process, starves it for CPU/Memory, or blocks its network access can cause this error.