##### SUMMARY
docker_image module is missing etc_hosts parameter even if this is documented and accepted by docker.build() as  of https://docker-py.readthedocs.io/en/stable/images.html

The same argument is accepted by docker_run but the lack of support here prevents us from building images when extra_hosts is needed.

I mention that the only reason why I propose using `etc_hosts` is in order to keep it in sync with already existing implementation from `docker_container` module which accepts `etc_hosts` which is translated to `extra_hosts` on docker-python-api.

I would personally prefer to use the same names as docker-api but this would require changing docker_container too.

##### ISSUE TYPE
- Bug Report

##### COMPONENT NAME
docker_image

##### ANSIBLE VERSION
<!--- Paste verbatim output from "ansible --version" between quotes -->
```paste below
ansible-devel
```

##### CONFIGURATION
<!--- Paste verbatim output from "ansible-config dump --only-changed" between quotes -->
```paste below

```

##### OS / ENVIRONMENT
<!--- Provide all relevant information below, e.g. target OS versions, network device firmware, etc. -->


##### STEPS TO REPRODUCE
<!--- Describe exactly how to reproduce the problem, using a minimal test-case -->

<!--- Paste example playbooks or commands between quotes below -->
```yaml

```

<!--- HINT: You can paste gist.github.com links for larger files -->

##### EXPECTED RESULTS
<!--- Describe what you expected to happen when running the steps above -->


##### ACTUAL RESULTS
<!--- Describe what actually happened. If possible run with extra verbosity (-vvvv) -->

<!--- Paste verbatim command output between quotes -->
```paste below

```

argument_spec = dict(
        source=dict(type='str', choices=['build', 'load', 'pull', 'local']),
        build=dict(type='dict', suboptions=dict(
            cache_from=dict(type='list', elements='str'),
            container_limits=dict(type='dict', options=dict(
                memory=dict(type='int'),
                memswap=dict(type='int'),
                cpushares=dict(type='int'),
                cpusetcpus=dict(type='str'),
            )),
            dockerfile=dict(type='str'),
            http_timeout=dict(type='int'),
            network=dict(type='str'),
            nocache=dict(type='bool', default=False),
            path=dict(type='path', required=True),
            pull=dict(type='bool'),
            rm=dict(type='bool', default=True),
            args=dict(type='dict'),
            use_config_proxy=dict(type='bool'),
            target=dict(type='str'),
        )),
        archive_path=dict(type='path'),
        container_limits=dict(type='dict', options=dict(
            memory=dict(type='int'),
            memswap=dict(type='int'),
            cpushares=dict(type='int'),
            cpusetcpus=dict(type='str'),
        ), removedin_version='2.12'),
        dockerfile=dict(type='str', removedin_version='2.12'),