Upgrade Rust from 1.39 to 1.43.1

Firecracker uses 1.43.1 since
firecracker-microvm/firecracker@2397f6a.

Signed-off-by: Kazuyoshi Kato <[email protected]>

Author: kzys

tools/docker/Dockerfile

@@ -11,7 +11,7 @@
 # express or implied. See the License for the specific language governing
 # permissions and limitations under the License.
 
-FROM rust:1.39-stretch
+FROM rust:1.43.1-buster
 
 ENV DEBIAN_FRONTEND="noninteractive"
 RUN apt-get update && apt-get install --yes --no-install-recommends \