HACKING: add steps for how to build the node image.

INSTALLER-HACKING.md

@@ -67,3 +67,42 @@ To reduce memory usage, you can remove some non-essential components from the cl
 ```
 export HACK_MINIMIZE=1
 ```
+
+### Developing with a local copy of openshift-sdn
+
+Origin images are difficult to build correctly. If you want to test some binary changes with openshift-sdn, there's an easier way.
+
+#### One-time setup:
+1. Set up your API credentials:
+    - Log on to https://api.ci.openshift.org/ with your GitHub credentials
+    - On the top right, click copy login command. Execute it in a terminal
+    - Execute `oc registry login`, which will install credentials for your local podman and docker clients
+2. Create the following dockerfile in your origin repo:
+```
+cat <<EOF > Dockerfile.node-hacking
+FROM docker.io/openshift/origin-node:v4.0.0
+COPY _output/local/bin/linux/amd64/openshift-sdn /usr/bin/openshift-sdn
+EOF
+```
+
+#### Building a development sdn image
+1.  Pick a registry. You can use the Openshift CI one if you are a member of the organization.
+```
+export REGISTRY=registry.svc.ci.openshift.org/<YOUR-GITHUB-USERNAME>
+```
+
+2. Build the sdn process
+```
+make WHAT=./cmd/openshift-sdn
+```
+
+3. Build and push the node image
+```
+podman build -t ${REGISTRY}/origin-node:latest -f Dockerfile.node-hacking .
+podman push ${REGISTRY}/origin-node:latest
+```
+
+4. Follow the steps above, but override the node image reference in step 3.
+```
+echo "NODE_IMAGE=${REGISTRY}/origin-node:latest" >> ${CLUSTER_DIR}/env.sh
+```