inv pod.shell deprecation warning

`inv pod.shell` throws a deprecation warning:

```
$ inv staging pod.shell
kubectl exec -it deploy/pressweb-web -n pressweb-staging bash
kubectl exec [POD] [COMMAND] is DEPRECATED and will be removed in a future version. Use kubectl exec [POD] -- [COMMAND] instead.
```

Search the project for the `inv pod.shell` task and the command that invokes `kubectl exec`. Update the invocation to use the documented `--` separator, then run the relevant task against a suitable cluster and confirm the deprecation warning is gone.

Written by the indexing model from the issue text.