diff --git a/libcxx/utils/ci/run-buildbot-container b/libcxx/utils/ci/run-buildbot-container
--- a/libcxx/utils/ci/run-buildbot-container
+++ b/libcxx/utils/ci/run-buildbot-container
@@ -12,6 +12,12 @@
 # state in `/llvm` is shared between the container and the host machine, which
 # is useful for editing files on the host machine and re-running the build bot
 # in the container.
+#
+# You will not be able to write to the mount from within the container
+# unless the user that created the monorepo matches "libcxx-builder".
+# If you need to do this, give that user permission to do so after running
+# the container. Or add this flag to run the container as your local user IDs:
+# --user $(id -u):$(id -g)
 
 set -e
 
@@ -21,4 +27,4 @@
     exit 1
 fi
 docker pull ldionne/libcxx-builder
-docker run -it --volume "${MONOREPO_ROOT}:/llvm" --workdir "/llvm" ldionne/libcxx-builder bash
+docker run -it --volume "${MONOREPO_ROOT}:/llvm" --workdir "/llvm" --cap-add=SYS_PTRACE ldionne/libcxx-builder bash