Correct docs for container_layer's mode attribute

[coryrc]

PR Checklist

Please check if your PR fulfills the following requirements:

• [N/A] Tests for the changes have been added (for bug fixes / features)
• [X] Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• [ ] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, local variables)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] CI related changes
• [x] Documentation content changes
• [ ] Other... Please describe:

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Other information

I have personally verified this. In addition, you can see the code uses the same file_attributes function which sets the mode.

[coryrc]

@rbielak