container: T6208: fix AttributeError: 'ConfigDict' object has no attribute 'upper' (backport #3272)

[mergify]

Change Summary

Commit b30faa4 (container: T6208: rename "cap-add" CLI node to "capability") added an AttributeError referencing an out of scope variable.

This has been fixed.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

• https://vyos.dev/T6208

Related PR(s)

• container: T6208: rename "cap-add" CLI node to "capability" #3269

Component(s) name

container

Proposed changes

How to test

run add container image busybox:stable
set container name c01 image busybox:stable
set container name c01 allow-host-networks
set container name c01 capability net-raw

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

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This is an automatic backport of pull request #3272 done by [Mergify](https://mergify.com).