Bump Docker version

Author: vitobotta

runner/Makefile

@@ -1,7 +1,7 @@
 NAME ?= summerwind/actions-runner
 
 RUNNER_VERSION ?= 2.168.0
-DOCKER_VERSION ?= 19.03.6
+DOCKER_VERSION ?= 19.03.8
 
 docker-build:
 	docker build --build-arg RUNNER_VERSION=${RUNNER_VERSION} --build-arg DOCKER_VERSION=${DOCKER_VERSION} -t ${NAME}:latest -t ${NAME}:v${RUNNER_VERSION} .