Helm chart: Configurable securityContext on container level

[hsudbrock]

The helm chart already permits to configure the securityContext on the pod level. However, in some cases it is also necessary to configure the securityContext on the container level. In particular, the fields made available by Kubernetes for the securityContext differ on pod and container level.

This issue is for making the securityContext also configurable on the container level.

Note: I intend to create a merge request to address this issue; not sure if adding an issue is needed in that case, I do so anyways, just for completeness.