Workarround missing ARG support in dependabot

Parrent image can't yet be parameterized for version parsing
dependabot/dependabot-core#2057

Dockerfile

@@ -6,12 +6,15 @@
 # docker build -t nav2:latest \
 #   --build-arg UNDERLAY_MIXINS \
 #   --build-arg OVERLAY_MIXINS ./
-ARG FROM_IMAGE=osrf/ros2:testing
 ARG UNDERLAY_WS=/opt/underlay_ws
 ARG OVERLAY_WS=/opt/overlay_ws
 
+# multi-stage for dependabot
+# https://github.com/dependabot/dependabot-core/issues/2057
+FROM osrf/ros2:testing-20210605003201 AS from_image
+
 # multi-stage for caching
-FROM $FROM_IMAGE AS cacher
+FROM from_image AS cacher
 
 # clone underlay source
 ARG UNDERLAY_WS
@@ -35,7 +38,7 @@ RUN find . -name "src" -type d \
       | xargs cp --parents -t /tmp/opt || true
 
 # multi-stage for building
-FROM $FROM_IMAGE AS builder
+FROM from_image AS builder
 
 # config dependencies install
 ARG DEBIAN_FRONTEND=noninteractive