Created guide for docker save command. (#18785)

* Created guide for docker save command.

* Updated subheading for consistency.

client/src/pages/guide/english/docker/docker-save/index.md

@@ -0,0 +1,22 @@
+---
+title: Docker save
+---
+
+## Docker save
+
+`docker save` stores a Docker image to a tar archive. It is useful to make an offline backup of an image or transfer an image to a computer in a corporate environment which does not have a direct access to the Internet.
+
+For example, to save the nginx image already in local computer to a file named `nginx-backup.tar`, the following command can be run:
+
+```
+docker save -o nginx-backup.tar nginx
+```
+
+Or with a specific version:
+
+```
+docker save -o nginx-backup.tar nginx:1.15.5
+```
+
+#### More Information:
+- [Docker CLI docs: save](https://docs.docker.com/engine/reference/commandline/save/)