In `adapters/http.js`, it request use `https` or `http` module with `options`.

```javascript
var options = {
  hostname: parsed.hostname,
  port: parsed.port,
  path: buildURL(parsed.path, config.params, config.paramsSerializer).replace(/^\?/, ''),
  method: config.method,
  headers: headers,
  agent: agent,
  auth: auth
};
``` 
We can add a key to support unix domain socket

```javascript
var options = {
  hostname: parsed.hostname,
  port: parsed.port,
  path: buildURL(parsed.path, config.params, config.paramsSerializer).replace(/^\?/, ''),
  socketPath: config.socketPath,  // just for test
  method: config.method,
  headers: headers,
  agent: agent,
  auth: auth
};
```

using axios like

```javascript
axios({
  socketPath: '/var/run/docker.sock',
  url: '/images/json'
})
.then(resp => console.log(resp.data))
```

I don't know if it fits your design concept, so I mentioned this issue

lib/adapters/http.js
};
        if (parsedProxyUrl.auth) {
          var proxyUrlAuth = parsedProxyUrl.auth.split(':');
          proxy.auth = {
            username: proxyUrlAuth[0],
            password: proxyUrlAuth[1]
          };
        }
      }
    }
    if (proxy) {
      options.hostname = proxy.host;
      options.host = proxy.host;
      options.headers.host = parsed.hostname + (parsed.port ? ':' + parsed.port : '');
      options.port = proxy.port;
      options.path = protocol + '//' + parsed.hostname + (parsed.port ? ':' + parsed.port : '') + options.path;
      
      if (proxy.auth) {
        var base64 = new Buffer(proxy.auth.username + ':' + proxy.auth.password, 'utf8').toString('base64');
        options.headers['Proxy-Authorization'] = 'Basic ' + base64;
      }
    }
    var transport;
    if (config.transport) {
      transport = config.transport;
    } else if (config.maxRedirects === 0) {
      transport = isHttps ? https : http;
    } else {
      if (config.maxRedirects) {
        options.maxRedirects = config.maxRedirects;