Environment Specs so far had only an image URL to specify a container image.
This was fine for public images but fell short in a few of cases:
(a) Using private image registries
(b) Specifying environment variables (this is needed for workflows helm install)
(c) Setting a SecurityContext for the container
This change adds the Container object to both build and runtime Environments.
Compatibility is preserved -- the existing ImageURL field is still used. See the comments in types.go for the overriding rules in the case that both Container and ImageURL are specified.
