Recover fix to testPullImage from commit 2a6d92da74343d7906ebfb0678eb7d3b1b8f5735, accidentally lost in 4ac45cd93ef94037331fee091ec6f004cf56ead8
Change how a tarball is generated when sending a build request. Instead of creating a temporary folder and copying all the files in a flat structure, read the files directly and preserve their path details.

Fixes a problem with adding files in a subfolder like:
Step 2 : ADD files/authorized_keys   /var/lib/jenkins/.ssh/

[Docker] ERROR: {"errorDetail":{"message":"files/authorized_keys: no such file or directory"},"error":"files/authorized_keys: no such file or directory"}
Merge pull request #3 from hackmann/build-add-file-in-subfolder

Build add file in subfolder