Docker plugin tries to create / delete the same network multiple times

When using the docker plugin, if multiple platforms are specified with the same network, the create and destroy playbooks loop through the same network multiple times, trying to create / delete the docker network. This doesn't cause any major problem, as the create / delete network operation is idempotent, but it's inefficient, especially when there are lots of platforms in the same molecule scenario, all sharing the same network.

A minimal example of the molecule.yml to produce this is below:
```yaml
---
dependency:
name: galaxy
driver:
name: docker
platforms:
- name: test1.local
image: alpine
pre_build_image: true
command: ${MOLECULE_DOCKER_COMMAND:-""}
networks:
- name: "my-test-network"
network_mode: "my-test-network"
- name: test2.local
image: alpine
pre_build_image: true
command: ${MOLECULE_DOCKER_COMMAND:-""}
networks:
- name: "my-test-network"
network_mode: "my-test-network"
provisioner:
name: ansible
verifier:
name: ansible
```

When `molecule create` is run with the above, it gives the following output (other output removed for clarity) - note that it calls the "create_network.yml" twice:
```
...
TASK [Create docker network(s)] ************************************************
included: /usr/local/lib/python3.9/site-packages/molecule_plugins/docker/playbooks/tasks/create_network.yml for localhost => (item=my-test-network)
included: /usr/local/lib/python3.9/site-packages/molecule_plugins/docker/playbooks/tasks/create_network.yml for localhost => (item=my-test-network)

TASK [Check if network exist] **************************************************
ok: [localhost]

TASK [Create docker network(s)] ************************************************
changed: [localhost]

TASK [Check if network exist] **************************************************
ok: [localhost]

TASK [Create docker network(s)] ************************************************
skipping: [localhost]
...
```

This could be easily solved by adding a `unique` filter to the `with_items` statements in `create.yml` and `destroy.yml` playbooks - i.e. in https://github.com/ansible-community/molecule-plugins/blob/main/src/molecule_plugins/docker/playbooks/create.yml#L90 , change:
```
with_items: "{{ molecule_yml.platforms | molecule_get_docker_networks(molecule_labels) }}"
```
to
```
with_items: "{{ molecule_yml.platforms | molecule_get_docker_networks(molecule_labels) | unique }}"
```

and in https://github.com/ansible-community/molecule-plugins/blob/main/src/molecule_plugins/docker/playbooks/destroy.yml#L49 , change
```
loop: "{{ molecule_yml.platforms | molecule_get_docker_networks() }}"
```
to
```
loop: "{{ molecule_yml.platforms | molecule_get_docker_networks() | unique }}"
```

Happy to submit a PR for this if helpful?

Read src/molecule_plugins/docker/playbooks/create.yml and destroy.yml, then run the minimal molecule.yml scenario with two platforms sharing my-test-network. The create and destroy playbooks should process each distinct network once rather than once per platform.

Written by the indexing model from the issue text.