Enable Proxy support

[shrids]

Currently com.spotify.docker.client.DefaultDockerClient does not provide a way to configure http proxy.
We could add something on the following lines so that the internal jaxws.rs client has proxy configured.

final ClientConfig config = DEFAULT_CONFIG
        .connectorProvider(new ApacheConnectorProvider())
        .property(ApacheClientProperties.CONNECTION_MANAGER, cm)
        .property(ApacheClientProperties.REQUEST_CONFIG, requestConfig)
        .property(ClientProperties.PROXY_URI, proxy);
        .property(ClientProperties.PROXY_USERNAME,user);
        .property(ClientProperties.PROXY_PASSWORD,pass);

[mattnworb]

Any idea if the Jersey/ApacheConnectorProvider will respect system properties like -Dhttp.proxyHost, like the `java.net. package does? If not that is a bummer.

Would you be willing to create a PR that adds these properties to the DefaultDockerClient.Builder class? that seems like the most natural place to configure this if the system properties aren't read by Jersey.

[shrids]

System properties like http.proxyHost are not read by Jersey. Added the following snippet inside the package private constructor of DefaultDockerClient.

    final String proxyHost = System.getProperty("http.proxyHost");
    if (proxyHost != null) {
      config.property(ClientProperties.PROXY_URI, proxyHost + ":"
              + System.getProperty("http.proxyPort", "8080"));
    }

This change ensured all the HTTP GET requests to dockerd works. But all POST requests to dockerd throws an EOF error (error snippet below) . Wondering if it is a setup issue or a proxy issue.
docker[21429]: time="2017-05-16T08:31:05.692109391Z" level=error msg="Handler for POST /containers/create returned error: EOF"

I have a docker daemon which supports 1.24 api version. dockerD version is 1.12.6

[shrids]

Found the reason why POST failed while using tinyproxy. (tinyproxy/tinyproxy#58)
To solve this we need to ensure content-length is populated by jersey client. This can be done by setting the following additional property in jersey client
config.property(ClientProperties.REQUEST_ENTITY_PROCESSING, RequestEntityProcessing.BUFFERED);
With this change it is working fine...