feat: connect containers to custom network

- Export the `get` function in `utils/env.js`
- Change the Docker image in `docker.js` from `ubuntu` to `lancatlin/quest-helloworld`
- Remove the `createContainer` and `getContainer` functions from `docker.js`
- Add a `network` constant in `docker.js` that gets the Docker network from the config
- Connect the container to the network in `getOrCreateContainer` function in `docker.js`
- Add `API_ENDPOINT` environment variable in the `attachContainer` function in `docker.js`

Signed-off-by: wancat <wancat@wancat.cc>

server/src/utils/env.js

@@ -0,0 +1,7 @@
+export function get(key, defaultValue) {
+  const value = process.env[key]
+  if (value != null) {
+    return value
+  }
+  return defaultValue
+}