Update docker.clj

When I run "lein uberimage" I get the following error:

"No such var: clj-http.util/force-byte-array, compiling:(com/palletops/docker.clj:69:14)"

I __think__ that this is because you have not required clj-http.util in your ns, and you have only required in some parts of the cli-http namespace, so your original use of cli-http.util on line 69 is causing the compile error above.

I have added a require for cli-http.util and updated the function call on line 69 appropriately.

Thanks in advance - apologies if I've missed something obvious; I have no idea how I could test this locally as if I were to meddle with the clj-docker .jar in my ~/.m2 repo I am pretty sure that lein would barf because the .sha would no longer match.
@hugoduncan Also, and I realise I am parading my ignorance here so apologies up-front, but how does this line work (if the correct version of Leiningen is used), considering that you have not "required" either the whole library or the .util namespace?  Does Leiningen require it on your behalf, and so it's available already?