Docs: Change path where to create the config.json file when using kaniko

What does this MR do?

This MR changes the path where to create the config.json file when building Docker images with kaniko, this is most useful when building multiple images in the same CI job taking advantage of the --cleanup new kaniko executor flag. Using that flag leads to the folder /root/.docker being deleted and therefore losing the config file. The /kaniko/.docker folder instead persists across cleanups and so there is no need to re-create the config file after each execution.

Related issues

Related to gitlab-org/gitlab-ce#50585

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards