The class NetworkConfig is missing a field corresponsing to "Attachable" attribute

[fdoemges]

According to the Docker Engine API (v1.26 and older as well) documentation a "network create" call (POST /networks/create) takes a JSON with a network config in the body, that contains the field "Attachable" which can be set to true in order to make the network attachable.

The corresponding message can be created with com.spotify.docker.client.messages.NetworkConfig.builder().build()

Unfortunately the class com.spotify.docker.client.messages.NetworkConfig doese not contain the field "Attachable" so this option cannot be used in the moment

[fdoemges]

Was fixed by #770