Revert "Disable sibling Docker feature"

Breaks bazel_docker_sandboxing_test and rbe_ubuntu1604 platform.

This reverts commit c396f4611750ce09a469aefb9a18f8c2cd863720.
diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index 4e0b561..d947b86 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -1769,6 +1769,7 @@
                     "/opt:/opt:ro",
                     "/var/lib/buildkite-agent:/var/lib/buildkite-agent",
                     "/var/lib/gitmirrors:/var/lib/gitmirrors:ro",
+                    "/var/run/docker.sock:/var/run/docker.sock",
                 ],
             }
         },