Missing `SecurityContext` for container

[SimonWoidig]

The imgproxy Helm chart doesn't support container-level SecurityContext. This prevents the usage of read-only filesystem for example as the pod security context and container security context are different. See the PodSecurityContext and ContainerSecurityContext.

It is loaded from values for the pod only: https://github.com/imgproxy/imgproxy-helm/blob/152c10713c0fc8daee7a29f386725910c2f673b5/imgproxy/templates/deployment.yaml#L68-L70

The solution is simple: add another field to the values named containerSecurityContext and use that to fill out the container security context.

I think I'll be able to provide this.