Use chunked encoding when passing the docker image.

If you don't do this, you very often run out of heap space, as the
webclient will create a buffer for the entire image file (possibly
many gigabytes) before sending it.

Signed-off-by: Nigel Magnay <nigel.magnay@gmail.com>
Merge pull request #118 from magnayn/pr2

Use chunked encoding when passing the docker image.