Run each build using docker exec, for faster builds

cibuildwheel/linux.py

@@ -100,11 +100,13 @@ def build(project_dir, output_dir, test_command, before_test, test_requires, tes
                             '--name', container_name,
                             '-i',
                             '-v', '/:/host',  # ignored on CircleCI
-                            docker_image, '/bin/bash'], check=True)
+                            docker_image], check=True)
             subprocess.run(['docker', 'cp',
                             os.path.abspath(project_dir) + '/.',
                             container_name + ':/project'], check=True)
 
+            subprocess.run(['docker', 'start', container_name])
+
             for config in platform_configs:
                 if dependency_constraints:
                     constraints_file = dependency_constraints.get_for_python_version(config.version)
@@ -113,7 +115,7 @@ def build(project_dir, output_dir, test_command, before_test, test_requires, tes
                                     container_name + ':/constraints.txt'], check=True)
 
                 subprocess.run(
-                    ['docker', 'start', '-i', '-a', container_name],
+                    ['docker', 'exec', '-i', container_name, '/bin/bash'],
                     universal_newlines=True,
                     check=True,
                     input='''