Use DOCKER_BUILDKIT=1 in CI

Author: dshemetov

.github/workflows/ci.yaml

@@ -4,6 +4,8 @@ on:
 jobs:
   build:
     runs-on: ubuntu-latest
+    env:
+      DOCKER_BUILDKIT: 1
     steps:
       - run: mkdir -p repos/undefx
       - name: Checkout undefx/py3tester