Upgrade Rust to 1.39

- Rust 1.35 cannot parse Firecracker's Cargo.lock somehow
- While Rust's latest release is 1.40.0, we'd like to keep in sync
  with Firecracker upstream.

firecracker-microvm/firecracker#1419

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

Author: kzys

tools/docker/Dockerfile.firecracker-builder

@@ -11,7 +11,7 @@
 # express or implied. See the License for the specific language governing
 # permissions and limitations under the License.
 
-FROM rust:1.35-stretch
+FROM rust:1.39-stretch
 
 ENV DEBIAN_FRONTEND="noninteractive"
 RUN apt-get update && apt-get install --yes --no-install-recommends \