In values.yaml you have nextcloud.extraSecurityContext but it's not used anywhere in the helm chart.

https://github.com/nextcloud/helm/blob/master/charts/nextcloud/values.yaml#L174

What is used is .securityContext

https://github.com/nextcloud/helm/blob/master/charts/nextcloud/templates/deployment.yaml#L445