add info on "how to create Dockerfiles" in step 5.1

[vedantbahel]

File: docker-hub/index.md

[nitishfy]

If you're using Linux, You can use the command: Touch Dockerfile and add the contents as shown there.

[participator]

Similar to #15944 and could use the same solution

[docker-robott]

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/lifecycle stale

[docker-robott]

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