Detects containedctx on library types

[DanielHilton]

I have recently enabled this check via golangci-lint, and come across situations where the check is failing for a dependency's struct.

https://github.com/grpc/grpc-go/blob/master/clientconn.go

The ClientConn type contains a context.Context, and when a struct I write includes an instance of a pointer to this type, the check fails. I can't change the type, it's not my code. So the linter should ignore it.