Docker driver schema rejects valid list-form command

The Docker driver schema added in `molecule-plugins` PR [#340](https://github.com/ansible-community/molecule-plugins/pull/340) rejects `platforms[].command` when it is supplied as a YAML list.

This configuration previously worked (in version 26.7.8):

```yaml
driver:
name: docker

platforms:
- name: namenode
image: example/hdfs:latest
command:
- hdfs
- namenode
```

Version 26.7.15 fails during Molecule configuration validation with an error similar to:

```text
ERROR Failed to validate molecule/default/molecule.yml

is not of type 'string'"]
```
(as a sidenote - this is not a helpful error message and requires commenting out line by line to identify the issue)

However, the underlying `community.docker.docker_container` module accepts `command` in either string or list form. List form is also useful because it preserves argument boundaries and avoids shell parsing and quoting issues.

## Steps to reproduce

1. Install a current version of Molecule and `molecule-plugins` with the Docker driver.
2. Create a scenario containing:

```yaml
driver:
name: docker

platforms:
- name: test
image: example/image:latest
command:
- hdfs
- namenode
```

3. Run:

```console
molecule syntax
```

## Actual behaviour

Molecule fails before execution because the Docker driver schema requires `command` to be a string.

## Expected behaviour

The Docker driver schema should accept either:

```yaml
command: "hdfs namenode"
```

or:

```yaml
command:
- hdfs
- namenode
```

Start by reproducing the failure with the Docker driver configuration and `molecule syntax`, then trace the Docker driver schema validation for `platforms[].command`. Done means both string and YAML list forms pass validation, while the existing string form continues to work.

Written by the indexing model from the issue text.