Add configuration item of container.network (#184)

Close https://gitea.com/gitea/act_runner/issues/177
Related https://gitea.com/gitea/act/pulls/56

### ⚠️ Breaking
The `container.network_mode` is a deprecated configuration item. It may be removed after Gitea 1.20 released.
Previously, if the value of `container.network_mode` is `bridge`, it means that `act_runner` will create a new network for job.But `bridge` is easily confused with the bridge network created by Docker by default.
We recommand that using `container.network` to specify the network to which containers created by `act_runner` connect.

###  🆕 container.network
The configuration file of `act_runner` add a new item of `contianer.network`.
In `config.example.yaml`:
```yaml
container:
  # Specifies the network to which the container will connect.
  # Could be host, bridge or the name of a custom network.
  # If it's empty, act_runner will create a network automatically.
  network: ""
```

As the comment in the example above says, the purpose of the `container.network` is specifying the network to which containers created by `act_runner` will connect.

`container.network` accepts the following valid values:
- `host`: All of containers (including job containers and service contianers) created by `act_runner` will be connected to the network named `host` which is created automatically by Docker. Containers will share the host’s network stack and all interfaces from the host will be available to these containers.
- `bridge`: It is similar to `host`. All of containers created by `act_runner` will be connected to the network named `bridge` which is created automatically by Docker. All containers connected to the `bridge` (Perhaps there are containers that are not created by `act_runner`) are allowed to communicate with each other, while providing isolation from containers which are not connected to that `bridge` network.
- `<custom_network>`: Please make sure that the `<custom_network>` network already exists firstly (`act_runner` does not detect whether the specified network exists currently. If not exists yet, will return error in the stage of `docker create`). All of containers created by `act_runner` will be connected to `<custom_network>`. After the job is executed, containers are removed and automatically disconnected from the `<custom_network>`.
- empty: `act_runner` will create a new network for each job container and their service containers (if defined in workflow). So each job container and their service containers share a network environment, but are isolated from others container and the Docker host. Of course, these networks created by `act_runner` will be removed at last.

### Others
- If you do not have special needs, we highly recommend that setting `container.network` to empty string (and do not use `container.network_mode` any more). Because the containers created by `act_runner` will connect to the networks that are created by itself. This point will provide better isolation.
- If you set `contianer.network` to empty string or `<custom_network>`, we can be access to service containers by `<service-id>:<port>` in the steps of job. Because we added an alias to the service container when connecting to the network.

Co-authored-by: Jason Song <i@wolfogre.com>
Reviewed-on: https://gitea.com/gitea/act_runner/pulls/184
Reviewed-by: Jason Song <i@wolfogre.com>
Co-authored-by: sillyguodong <gedong_1994@163.com>
Co-committed-by: sillyguodong <gedong_1994@163.com>

internal/pkg/config/config.example.yaml

@@ -42,8 +42,10 @@ cache:
   port: 0
 
 container:
-  # Which network to use for the job containers. Could be bridge, host, none, or the name of a custom network.
-  network_mode: bridge
+  # Specifies the network to which the container will connect.
+  # Could be host, bridge or the name of a custom network.
+  # If it's empty, act_runner will create a network automatically.
+  network: ""
   # Whether to use privileged mode or not when launching task containers (privileged mode is required for Docker-in-Docker).
   privileged: false
   # And other options to be used when the container is started (eg, --add-host=my.gitea.url:host-gateway).