Try setting parallelism

Author: mhucka

.github/workflows/ci_docker_tests.yaml

@@ -71,9 +71,14 @@ jobs:
           driver: docker
 
       - name: Build Docker images
+        env:
+          # On GitHub, docker compose tries to use multiple concurrent calls.
+          # This causes the build to fail, because the test images depend on
+          # qsim-base, and when buildx can't find qsim-base, it tries to get
+          # it from docker.io.
+          COMPOSE_PARALLEL_LIMIT: 1
         run: |
-          docker compose build qsim-base-image
-          docker compose build qsim-cxx-tests-image qsim-py-tests-image
+          docker compose build
 
       - name: Run C++ tests
         run: docker run --rm qsim-cxx-tests:latest