Unable to use script tag inside containerset

### Pre-requisites

- [X] I have double-checked my configuration
- [X] I can confirm the issues exists when I tested with `:latest`
- [ ] I'd like to contribute the fix myself (see [contributing guide](https://github.com/argoproj/argo-workflows/blob/master/docs/CONTRIBUTING.md))

### What happened/what you expected to happen?

Hi Team

I would like to execute a python script using script tag inside a container of container set template.
But when I do it, while running, the values of image, args and command are read as empty strings as all the values are defined in script tag.

Can you please let me know how to use script tag along with containers in container set template.

Below is the error:

```
failed took-up entrypoint/cmd for image you must either explicitly specify the command, or list the image's command in the index: https://argoproj.github.io/argo-workflows/workflow-executors/#emissary-emissary: could not Parse reference:
```

Thanks
Mahesh

### Version

>= 3.1.0

### Paste a small workflow that reproduces the issue. We must be able to run the workflow; don't enter a workflows that uses private images.

```YAML
apiVersion: argoproj.io/v1alpha1
kind: Workflow
metadata:
generateName: container-set-template-
spec:
entrypoint: main
templates:
- name: main
volumes:
- name: workspace
emptyDir: { }
containerSet:
volumeMounts:
- mountPath: /workspace
name: workspace
containers:
- name: a
script:
image: python:alpine3.6
command: [python]
source: |
import random
i = random.randint(1, 100)
print(i)
- name: b
image: argoproj/argosay:v2
- name: main
image: argoproj/argosay:v2
dependencies:
- a
- b
```

### Logs from the workflow controller

```text
kubectl logs -n argo deploy/workflow-controller | grep ${workflow}
```

### Logs from in your workflow's wait container

```text
kubectl logs -n argo -c wait -l workflows.argoproj.io/workflow=${workflow},workflow.argoproj.io/phase!=Succeeded
```

Start by running the supplied containerSet workflow with its Python script and compare the generated container values with the script fields. Trace the containerSet handling for the nested script configuration; done means the script container receives its image and command and the workflow completes without the empty-image parsing error.

Written by the indexing model from the issue text.