Speed up build command

[cswinter]

The imagebuild example is much slower than the docker cli. For a relatively small build (docker cli reports 25.15MB of build context) which is fully cached, the docker cli takes around 0.27 seconds and imagebuild.rs takes 0m1.352s. I haven't profiled it, but output takes much longer to appear for imagebuild.rs and my suspicion is that the tarball operation might be the cause:

shiplift/src/image.rs

Line 124 in 9750d29

let tar_result = tarball::dir(&mut bytes, opts.path.as_str());