[Encore] Add version support for the --public parameter and a small docker support section

[shadowc]

Reflects changes in: symfony/webpack-encore#1095

Adds the version number at which this feature will be released, and it also adds a docker example for a nodejs box.

• Test docker setup

[louismariegaborit]

Could you update too the block example with a custom domain ?
To use HMR with custom domain, we must add --client-web-socket-url.
In the case of the doc, the option is --client-web-socket-url ws://app.vm:8080/ws and wss if https is enabled.

[shadowc]

I think the doc only talks about "serving your files" from a custom domain (not having a custom HMR url), but we can definitely make that distinction and let people know about this other option!