cros-test: install attr

setfattr is required for CTS (STS specifically).
attr + libattr1 is about 215kB on buster.

BUG=b:296797006
TEST=CQ

Change-Id: Icaf5ab306bfb16f65c6cafc084a60a00e97f6270
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/4795712
Tested-by: Shao-Chuan Lee <shaochuan@chromium.org>
Reviewed-by: Derek Beckett <dbeckett@chromium.org>
Commit-Queue: Shao-Chuan Lee <shaochuan@chromium.org>
(cherry picked from commit c6cad3c9dc554a317c9814674b4af4b3695b33a2)
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/4798310
Auto-Submit: Shao-Chuan Lee <shaochuan@chromium.org>
Commit-Queue: Derek Beckett <dbeckett@chromium.org>

src/chromiumos/test/dockerfiles/cros-test/Dockerfile

diff --git a/src/chromiumos/test/dockerfiles/cros-test/Dockerfile b/src/chromiumos/test/dockerfiles/cros-test/Dockerfile
index cb08f40..2d4a28d 100644
--- a/src/chromiumos/test/dockerfiles/cros-test/Dockerfile
+++ b/src/chromiumos/test/dockerfiles/cros-test/Dockerfile
@@ -4,10 +4,11 @@
 
 FROM python:3.8-slim-buster AS build
 
-# unzip, openjdk are for CTS specifically.
+# attr, unzip, openjdk are for CTS specifically.
 # squashfs-tools is for Uprev specifically.
 RUN apt-get update \
     && apt-get install -y \
+      attr \
       ca-certificates \
       curl \
       iproute2 \