documentation for docker users : permissions to bind mounts

What does this MR do?

This merge request add documentation for docker user of gitlab to know how to avoid Failed to bind mount errors.

No new page, just a new paragraph.

Related issues

The bug was reported by the issue gitlab-pages#129 (closed) and corrected by the MR gitlab-pages!92 (merged)

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
• Feature moving tiers? Make sure the change is also reflected in features.yml
• 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

/label ~Documentation