Add containerSecurityContext

This was originally raised in the auto-deploy-app repo, but I don't think it has still been fixed in this repo (now that it has been moved)

We could really do with this feature

Implementation Guide

An initial attempt to introduce this got stale. It got very close to getting merged. It just has some open questions that need addressing. Anyone is welcome to pick this up and push it forward, propose a different approach.

!293 (closed)

Update: Pod security context was implemented on !357 (merged).

We still don't support containerSecurityContext.