Make Docker version extraction more robust

scope

@@ -35,7 +35,8 @@ shift 1
 MIN_DOCKER_VERSION=1.5.0
 
 check_docker_version() {
-    if ! DOCKER_VERSION=$(docker -v | grep -oE "[0-9]+\.[0-9]+\.[0-9]+") || [ -z "$DOCKER_VERSION" ] ; then
+    if ! DOCKER_VERSION=$(docker -v | sed -n 's%^Docker version \([0-9]\{1,\}\.[0-9]\{1,\}\.[0-9]\{1,\}\).*$%\1%p') ||
+       [ -z "$DOCKER_VERSION" ] ; then
         echo "ERROR: Unable to parse docker version" >&2
         exit 1
     fi