Ubuntu: Add docker-buildx-plugin package

mbed-tls-restricted-rp-ci-testing job error, due to
the lack of docker-buildx-plugin package [1]

[1]: https://ci.trustedfirmware.org/view/Mbed-TLS/job/mbed-tls-restricted-pr-ci-testing/view/change-requests/job/PR-758-merge/lastFailedBuild/console#:~:text=55%3A18%20%20%2B-,DOCKER_BUILDKIT%3D1%20docker%20build%20%2D%2Dbuild%2Darg%20BUILDKIT_INLINE_CACHE%3D1,-%2D%2Dbuild%2Darg%20ARMLMD_LICENSE_FILE

Signed-off-by: Arthur She <arthur.she@linaro.org>
Change-Id: Ia59b008ea0d2a3d24b32994069a6a46c2c7997a4
diff --git a/ubuntu.sh b/ubuntu.sh
index 39c6fa3..9d3909c 100755
--- a/ubuntu.sh
+++ b/ubuntu.sh
@@ -23,7 +23,8 @@
 apt-get -qq update
 DEBIAN_FRONTEND=noninteractive apt-get install -qy --no-install-recommends \
     docker-ce \
-    docker-ce-cli
+    docker-ce-cli \
+    docker-buildx-plugin
 
 update-alternatives --install /usr/bin/python python /usr/bin/python3 1