Docker plugin does not expand variable for "privileged"

I'd like to pass a variable for the `privileged` part e.g.,

```yaml
platforms:
- name: arch-instance
image: arch-ansible
platform: ${DOCKER_DEFAULT_PLATFORM:-linux/amd64}
dockerfile: ../docker/Dockerfile
build_image: true
...
privileged: ${DOCKER_PRIVILEGED:-False}
```

During `TASK [Discover local Docker images]` the variable seems to be expanded correctly:

```
TASK [Discover local Docker images] ********************************************
ok: [localhost] => (item={'diff': [], '... 'platform': 'linux/amd64', 'privileged': False, ...
```

But then it fails later:

```
TASK [Wait for instance(s) creation to complete] *******************************
failed: ...
"msg": "argument 'privileged' is of type and we were unable to convert to bool:
The value '${DOCKER_PRIVILEGED:-False}' is not a valid boolean.
Valid booleans include: 'on', 'yes', 1, 0, '0', 'false', 'no', '1', 'f', 'off', 'y', 'true', 't', 'n'"...
```

Start by reproducing the Docker plugin configuration shown in the issue and trace the handling of `privileged` between `Discover local Docker images` and `Wait for instance(s) creation to complete`. Done means `${DOCKER_PRIVILEGED:-False}` is resolved to a boolean before the latter task, while preserving the existing behavior for literal boolean values.

Written by the indexing model from the issue text.