Docker command template

IMHO default docker command template is incorrect. Current template in version 4.1.12 is
```sh
docker run --rm -it --mount type=bind,source="{cwd}",target=/src backstopjs/backstopjs:{version} {backstopCommand} {args}
```
This template generate the following command:
```sh
docker run --rm -it --mount type=bind,source="/project_dir",target=/src backstopjs/backstopjs:4.1.12 test "--moby" "--network" "host"
```
In order to properly define `--network host` the docker command template needs to look like this:
```sh
docker run --rm -it --mount type=bind,source="{cwd}",target=/src --network host backstopjs/backstopjs:{version} {backstopCommand} {args}
```
With this docker command template, we can access running storybook instances on the host. Before this is resolved people can override the default docker command template by setting `dockerCommandTemplate` in backstop configuration file.