Add workflow step to build navigation2

Author: mkhansenbot

.github/workflows/docker-build.yml

@@ -34,6 +34,15 @@ jobs:
           cache-to: type=gha,mode=max
           no-cache: false
           outputs: type=docker,dest=/tmp/moveit2.tar
+      - name: Build nav2 image
+        uses: docker/build-push-action@v2
+        with:
+          context: navigation2
+          push: false
+          tags: osrf/space_nav2:latest
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
+          no-cache: false
       - name: Build space robots demo image
         run: |
           docker load --input /tmp/moveit2.tar