Update Debian version to 7.7

Replaces #9724

fix by: panticz <mail@konczalski.de> (github: panticz)
Signed-off-by: Sven Dowideit <SvenDowideit@docker.com>
Merge pull request #9782 from SvenDowideit/pr_out_update_debian_version_to_7_7

Update Debian version to 7.7