fix(docker): create s6 envdir before browser path export (#34601)

tests/tools/test_stage2_hook_puid_pgid.py

@@ -84,3 +84,18 @@ def test_hermes_uid_gid_take_precedence_over_aliases(stage2_text: str) -> None:
 def test_no_uid_vars_leaves_values_empty(stage2_text: str) -> None:
     # An empty resolution means the stage2 hook keeps the default hermes user.
     assert _resolve(stage2_text, {}) == ":"
+
+
+def test_stage2_hook_creates_s6_envdir_before_writing_browser_path(stage2_text: str) -> None:
+    """Regression guard for browser-path export on runtimes where the
+    s6 container_environment directory is absent when the cont-init hook runs.
+    """
+    mkdir_line = "mkdir -p /run/s6/container_environment"
+    write_line = (
+        "printf '%s' \"$browser_bin\" > "
+        "/run/s6/container_environment/AGENT_BROWSER_EXECUTABLE_PATH"
+    )
+
+    assert mkdir_line in stage2_text
+    assert write_line in stage2_text
+    assert stage2_text.index(mkdir_line) < stage2_text.index(write_line)