build: pull base image for --checkrebuild

build.py

@@ -107,6 +107,10 @@ def red(text):
 
 
 def build(rebuild=False):
+    if rebuild:
+        # pull base images
+        check_call('docker', 'pull', 'debian:stretch-slim')
+        check_call('docker', 'pull', 'debian:buster-slim')
     for matrix in MATRIX:
         for python, dist, cw, onbuild in itertools.product(*matrix):
             build_image(python, dist, cw, onbuild)