Add Milvus offline installation README (#7026)

Signed-off-by: Nquicksilver <zhifeng.zhang@zilliz.com>

deployments/offline/README.md

+# Milvus offline installation
+
+## Manually downloading Docker images
+
+Your Milvus installation may fail when images are not properly loaded from public Docker registries. To pull all images and save them into a directory that can be moved to the target host and loaded manually, perform the following procedure:
+
+1. Save Milvus manifest and Docker images
+
+If you install your Milvus with the **docker-compose.yml** file, use these command:
+
+- Download Milvus docker-compose.yaml
+```shell
+# Download Milvus standalone docker-compose.yaml
+wget https://raw.githubusercontent.com/milvus-io/milvus/master/deployments/docker/standalone/docker-compose.yml -O docker-compose.yml
+```
+
+or
+
+```shell
+# Download Milvus cluster docker-compose.yaml
+wget https://raw.githubusercontent.com/milvus-io/milvus/master/deployments/docker/cluster/docker-compose.yml -O docker-compose.yml
+```
+
+- Pull and save Docker images
+```shell
+# Pull and save Docker images
+pip3 install -r requirements.txt
+python3 save_image.py --manifest docker-compose.yml
+```
+
+If you install your Milvus with **Helm**, use these command: 
+- Update Helm repo
+```shell
+helm repo add milvus https://milvus-io.github.io/milvus-helm/
+helm repo update
+```
+
+- Get Kubernetes manifest of Milvus standalone
+```shell
+# Get Kubernetes manifest of Milvus standalone
+helm template my-release milvus/milvus > milvus_manifest.yaml
+```
+
+or
+
+```shell
+# Get Kubernetes manifest of Milvus cluster
+helm template --set cluster.enabled=true my-release milvus/milvus > milvus_manifest.yaml
+```
+
+- Pull and save Docker images
+```shell
+pip3 install -r requirements.txt
+python3 save_image.py --manifest milvus_manifest.yaml
+```
+
+The Docker images will be stored under **images** directory.
+
+2. Enter the following command to load the Docker images:
+
+```shell
+for image in $(find images/ -type f -name "*.tar.gz") ; do gunzip -c $image | docker load; done 
+```
+
+## Install Milvus
+
+- Install Milvus with Docker Compose
+
+```shell
+docker-compose -f docker-compose.yaml up -d
+```
+
+- Install Milvus on Kubernetes
+
+```shell
+kubectl apply -f milvus_manifest.yaml
+```
\ No newline at end of file

deployments/offline/save_image.py

 import argparse
 import docker
 import gzip
+import os
 import yaml
 from nested_lookup import nested_lookup
 
 
 if __name__ == "__main__":
     parser = argparse.ArgumentParser(
-        description="Save Docker image")
+        description="Save Docker images")
 
     parser.add_argument("--manifest",
                         required=True,
                         help="Path to the manifest yaml")
+    parser.add_argument("--save_path",
+                        type=str,
+                        default='images',
+                        help='Directory to save images to')
     arguments = parser.parse_args()
 
-
     with open(arguments.manifest, 'r') as file:
         template = file.read()
-    
+
     images=[]
     parts = template.split('---')
     for p in parts:
@@ -25,11 +29,15 @@ if __name__ == "__main__":
         matches = nested_lookup("image", y)
         if (len(matches)):
                 images += matches
-    
+
+    save_path = arguments.save_path
+    if not os.path.isdir(save_path):
+        os.mkdir(save_path)
+
     client = docker.from_env()
     for image_name in set(images):
         file_name = (image_name.split(':')[0].replace("/", "-"))
-        f = gzip.open( file_name + '.tar.gz', 'wb')
+        f = gzip.open(save_path + "/" + file_name + '.tar.gz', 'wb')
         try:
             image = client.images.get(image_name)
             if image.id:
@@ -40,3 +48,5 @@ if __name__ == "__main__":
         image_tar = image.save(named=True)
         f.writelines(image_tar)
         f.close()
+
+    print("Save docker images to \"" + save_path + "\"")