Add instructions to use the dockercoins/ images

We have images on the Docker Hub for the various components
of dockercoins. Let's add one slide explaining how to use that,
for people who would be lost or would have issues with their
registry, so that they can catch up.

slides/k8s/ourapponkube.md

@@ -246,6 +246,27 @@ class: extra-details
 
 ---
 
+## Catching up
+
+- If you have problems deploying the registry ...
+
+- Or building or pushing the images ...
+
+- Don't worry: we provide pre-built images hosted on the Docker Hub!
+
+- The images are named `dockercoins/worker:v0.1`, `dockercoins/rng:v0.1`, etc.
+
+- To use them, just set the `REGISTRY` environment variable to `dockercoins`:
+  ```bash
+  export REGISTRY=dockercoins
+  ```
+
+- Make sure to set the `TAG` to `v0.1`
+
+  (our repositories on the Docker Hub do not provide a `latest` tag)
+
+---
+
 ## Deploying all the things
 
 - We can now deploy our code (as well as a redis instance)