Fix a Pylint warning

cibuildwheel/docker_container.py

@@ -128,7 +128,7 @@ class DockerContainer:
                 cwd=from_path,
             )
         else:
-            docker = subprocess.Popen(
+            with subprocess.Popen(
                 [
                     "docker",
                     "exec",
@@ -139,7 +139,7 @@ class DockerContainer:
                     f"cat > {shell_quote(to_path)}",
                 ],
                 stdin=subprocess.PIPE,
-            )
+            ) as docker:
                 docker.stdin = cast(IO[bytes], docker.stdin)
 
                 with open(from_path, "rb") as from_file: