Add to air-gapped doc how to use a mirrored operator image (#7019)

Add a chapter how to run the ECK operator itself in an air-gapped environment.

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Co-authored-by: Peter Brachwitz <peter.brachwitz@elastic.co>
Co-authored-by: Thibault Richard <thb.krkr@gmail.com>

docs/operating-eck/air-gapped.asciidoc

@@ -33,6 +33,14 @@ To make use of your mirrored images you can either set the image for each applic
 
 
 
+[float]
+[id="{p}-use-mirrored-operator-image"]
+== Use a mirrored image of the ECK operator
+
+To deploy the ECK operator in an air-gapped environment, you first have to mirror the operator image itself from `docker.elastic.co` to a private container registry, for example `my.registry`.
+
+Once the ECK operator image is copied internally, replace the original image name +docker.elastic.co/eck/eck-operator:{eck_version}+ with the private name of the image, for example +my.registry/eck/eck-operator:{eck_version}+, in the <<{p}-install-yaml-manifests,operator manifests>>. When using <<{p}-install-helm,Helm charts>>, replace the `image.repository` Helm value with, for example, `my.registry/eck/eck-operator`.
+
 [float]
 [id="{p}-container-registry-override"]
 == Override the default container registry