[installer] Add air-gap instructions

/werft no-preview

Author: corneliusludmann

install/installer/docs/air-gap.md

@@ -0,0 +1,37 @@
+# Installing Gitpod in an air-gap network with the Gitpod Installer
+
+## Mirror Gitpod Images
+
+You need a registry that is reachable in your network. Add the domain of your registry to the Gitpod config `gitpod.config.yaml` like this:
+```yaml
+repository: your-registry.example.com
+```
+
+The command `gitpod-installer mirror list` gives you a list of all images needed by Gitpod. You can run the following code to pull the needed images and push them to your registry:
+
+```
+for row in $(gitpod-installer mirror list --config gitpod.config.yaml | jq -c '.[]'); do
+    original=$(echo $row | jq -r '.original')
+    target=$(echo $row | jq -r '.target')
+
+    docker pull $original
+    docker tag $original $target
+    docker push $target
+done
+```
+
+## Install Gitpod in Air-Gap Mode
+
+To install Gitpod in an air-gap network, you need to configure the repository of the images needed by Gitpod (see previous step) and disable the `definitely-gp` feature. Add this to your Gitpod config:
+
+```yaml
+repository: your-registry.example.com
+disableDefinitelyGp: true
+```
+
+That's it. Run the following commands as usual and Gitpod fetches the images from your registry and does not need internet access to operate:
+
+```
+gitpod-installer render --config gitpod.config.yaml > gitpod.yaml
+kubectl apply -f gitpod.yaml
+```