Fix (container) securityContext for admin

[ntnn]

If the containerSecurityContext is set it results in two securityContext keys which causes the helm chart to fail.

      Helm upgrade failed for release mailu/mailu with chart [email protected]: error while running post render on files: map[string]interface {}(nil): yaml: unmarshal errors:
        line 111: mapping key "securityContext" already defined at line 52

[Copilot]

Pull Request Overview

This PR fixes a Helm chart deployment issue where duplicate securityContext keys were causing YAML parsing errors. The fix consolidates the container security context configuration to prevent conflicts between default and user-defined security contexts.

• Removes the conditional securityContext block that was causing duplication
• Merges user-defined container security context with the required NET_BIND_SERVICE capability
• Uses Helm template functions to properly combine security context configurations

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