tox.ini (docker): Use BUILDKIT_PROGRESS=plain to restore full output when DOCKER_BUILDKIT=1

Author: Matthias Koeppe

tox.ini

@@ -684,6 +684,9 @@ setenv =
 
     sitepackages:                              IGNORE_MISSING_SYSTEM_PACKAGES=yes
 
+    # https://docs.docker.com/engine/reference/commandline/buildx_build/#progress
+    docker:        BUILDKIT_PROGRESS=plain
+
 # environment will be skipped if regular expression does not match against the sys.platform string
 platform =
     local-macos:   darwin