[feature request] image build args

[cylewitruk]

Putting in my vote for build args as it's a TODO in the code 😃

docker-api-rs/src/opts/image.rs

Line 317 in b1f1891

// TODO: buildargs

[cylewitruk]

Looks like it was as simple as:

impl_map_field!(url
    /// Set build-time variables.
    build_args => "buildargs"
);

Works when I use the crate locally with that change, anyway. If I get some time to figure out how to write a decent test for this I'll PR it.