Add default container securityContext

[JJGadgets]

Details

Describe the solution you'd like:

Since there is a defaultPodOptions.securityContext in app-template v2.0, it would be nice to have a defaultContainerSecurityContext or similar so that a standardized securityContext can be applied to all containers in a HelmRelease using app-template.

This would be helpful for e.g. meeting restricted pod-security policy.

[JJGadgets]

As noted in Discord: more consideration and discussion would be needed, for factors such as how global should this be scoped (e.g. all containers in all controllers, all containers in a given controller, etc).

[onedr0p]

A possible solution could be what I recommended in #254 (comment) ?

I think it should apply to all containers in all controllers, since that is the behavior of defaultPodOptions