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.
The `clj-http.util` namespace is required by `clj-http.core`.  We should probably not be relying on this though, so adding the require makes sense.