Skip remote docker build result copy if the directory is mapped.

This patch fixes build issue with recent boot2docker release.

Before this patch, the build result copy was forced even if the
build directory on boot2docker VM was mapped to host.
This copy process involved rm-rf ing the target directory, which
auso rm-ed the build result if the directory was mapped to hsot.
This patch adds a check if the build directory is mapped to host,
and skips build result copy if not needed.

This resolves issue https://github.com/GoogleCloudPlatform/kubernetes/issues/1856