Add manage-offline-container-images.sh (#7024)

One challenge of offline deployment was how to collect necessary
container images as a preparation. This adds a script to solve it.

contrib/offline/README.md

@@ -0,0 +1,21 @@
+# Container image collecting script for offline deployment
+
+This script has two features:
+(1) Get container images from an environment which is deployed online.
+(2) Deploy local container registry and register the container images to the registry.
+
+Step(1) should be done online site as a preparation, then we bring the gotten images
+to the target offline environment.
+Then we will run step(2) for registering the images to local registry.
+
+Step(1) can be operated with:
+
+```shell
+$ ./manage-offline-container-images.sh   create
+```
+
+Step(2) can be operated with:
+
+```shell
+$ ./manage-offline-container-images.sh   register
+```