Add new customizations doc

doc/customizations-image.md

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+## Customizations image
+
+### Build
+
+To create a new Docker image containing our Gitea customizations (label sets,
+styles, page content, etc.):
+
+Edit `packer/custom.json` to increment the tag, then run this script (needs
+[Packer](https://www.packer.io/) in your path)
+
+    ./script/build_customizations_image
+
+### Deploy
+
+Edit `kubernetes/gitea-server.yaml` to use the new tag
+(`image: eu.gcr.io/fluted-magpie-218106/gitea_custom:$VERSION`) and apply the
+change:
+
+    cd kubernetes
+    kubectl apply -f gitea-server.yaml