resources/docker_files/build.sh broken by multi-arch support in list-docker-image-tags.sh

[gilles-peskine-arm]

Since 268b36e, .resources/docker_files/list-docker-image-tags.sh HASH lists both the amd64 and the arm64 image name. But build.sh expects it to output a single name. This breaks build.sh:

./build.sh ubuntu-22.04/Dockerfile 
[+] Building 0.0s (0/0)                                          docker:default
ERROR: invalid tag "ubuntu-22.04-d64006948b8db01a00a349fb4163c20550feaa1e-amd64\nubuntu-22.04-d64006948b8db01a00a349fb4163c20550feaa1e-arm64": invalid reference format