Exec image wiring (#3238)

* Example on how to wire an image via exec mixin

Signed-off-by: John Cudd <jmcudd@gmail.com>

* Small tweak to naming

Signed-off-by: John Cudd <jmcudd@gmail.com>

---------

Signed-off-by: John Cudd <jmcudd@gmail.com>

docs/content/docs/references/examples/airgap.md

@@ -85,6 +85,20 @@ Porter handles tracking the image location for you, just use the template variab
 
 [images]: /docs/bundle/manifest/#images
 
+If using exec mixin, image digests can be passed via environmental variables or arguments.
+
+```yaml
+install:
+  - exec:
+      description: "Insall WhaleGap"
+      command: ./porter-scripts.sh
+      arguments:
+        - install
+      envs:
+        IMAGE_whalesayd: "${ bundle.images.whalesayd.repository }@${ bundle.images.whalesayd.digest }"
+```
+
+
 ## Move the bundle across the airgap
 
 Let's simulate moving the bundle across an airgap by publishing the bundle to a different registry.