Fix doc error in Getting Started Supply Chain Security guide

[mbpavan]

Changes

Couple of small changes to the documentation for the Getting Started Supply Chain Security guide

1. Fix broken link for co-sign.
2. Pipeline run was failing with [kaniko-build : build-and-push] error building image: parsing dockerfile: Dockerfile parse error line 3: unknown instruction: EOF for create-dockerfile task. Added $(workspaces.source.path)/ for docker file creation

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[linux-foundation-easycla]

• ✅login: PavanBheeman / (d92c307)

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[AlanGreene]

Thanks for the PR @mbpavan. Please sign the CLA by following the instructions above so that this can be merged.

[PavanBheeman]

Thanks for the PR @mbpavan. Please sign the CLA by following the instructions above so that this can be merged.

Thanks, I have signed it. please review it

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