Set CIBUILDWHEEL environment variable to 1 during build

cibuildwheel/__main__.py

@@ -75,6 +75,10 @@ def main():
 
     skip = BuildSkipper(skip_config)
 
+    # Add CIBUILDWHEEL environment variable
+    # This needs to be passed on to the docker container in linux.py
+    os.environ['CIBUILDWHEEL'] = '1'
+
     try:
         project_setup_py = os.path.join(project_dir, 'setup.py')
         name_output = subprocess.check_output([sys.executable, project_setup_py, '--name'],

cibuildwheel/linux.py

@@ -111,6 +111,8 @@ def build(project_dir, package_name, output_dir, test_command, test_requires, be
         docker_process = subprocess.Popen([
                 'docker',
                 'run',
+                '--env',
+                'CIBUILDWHEEL',
                 '--rm',
                 '-i',
                 '-v', '%s:/project' % os.path.abspath(project_dir),