force the read of the tarSum so that sums actually get computed

Docker-DCO-1.1-Signed-off-by: Brice Jaglin <bjaglin@teads.tv> (github: bjaglin)
Remove the mtime for temp file. Prevent false negative cache

Docker-DCO-1.1-Signed-off-by: Guillaume J. Charmes <guillaume@charmes.net> (github: creack)
add test

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)
Merge pull request #5951 from vieux/pr5919

Fix remote add cache