Created guide for Docker pull command. (#18779)

* Created guide for Docker pull command.

* Updated subheading for consistency.

* Update index.md

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

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+---
+title: Docker pull
+---
+
+## Docker pull
+
+`docker pull` downloads images from Docker Hub or other Docker registries.
+
+For example, to download the latest nginx image, the command should be run like this:
+
+```
+docker pull nginx
+```
+
+This intrinsically assumes the image associated with `latest` tag. To download a specific version, the version info must be added to the image name with a colon:
+
+```
+docker pull nginx:1.15.5
+```
+
+Docker images can also be downloaded from registries other than Docker Hub by adding the registry address in front of image name:
+
+```
+docker pull myregistry.example.com/myapplicationimage
+```
+
+Or with a specific version:
+
+```
+docker pull myregistry.example.com/myapplicationimage:1.2.11
+```
+
+#### More Information:
+- [Docker CLI docs: pull](https://docs.docker.com/engine/reference/commandline/pull/)