Allow relative sockjs URL

[trashpanda001]

What problem does this feature solve?

I am using Nginx in development to terminate SSL and proxy to the dev-server (https://nginx/ => http://devserver:8080/) with devServer.public set appropriately, so I can use HTTPS/HTTP2 during development.

This had been working fine until beta16 commit:

2cbe373

which makes sockjs URL absolute with the wrong host/port, now in this case: https://10.0.1.6:8080/sockjs-node. Previously it was simply /sockjs-node.

I would like an option to either specify sockjsUrl explicitly OR an option to omit the protocol/host/port and just use the pathname /sockjs-node.

What does the proposed API look like?

module.exports = {
   devServer: {
      disableHostCheck: true,
      public: 'local.mox.cc',
      sockjsRelative: true, // OR
      sockjsUrl: '/sockjs-node'
   }
}

[trashpanda001]

I suppose building sockjsUrl from the devServer's public might work as well like //local.mox.cc/sockjs-node.

[yyx990803]

/cc @jkzing

[JohnCampionJr]

I think this would be useful, not sure why we wouldn't want this to be able to be relative
Relative sockjsUrl would allow for HMR to work whether the app is loaded directly from the webpackdevserver or through a proxied server on a different port.

[sokolovstas]

@sickp How to use devServer.public to build relative url?