reproducible_builds: ensure that umask doesn't affect the container build #3

• git doesn't track file permissions, thus
• git clone initiates the permissions according the user's umask value
• docker build just copies the files as-is into container
• especial problems are with root/usr directory created by ADD root / command, this affects the whole image (/usr directory might be unreadable)

In RPMs' specfiles, we explicitly set the permissions in %install phase (or by %attr in %files), though there's no such way in Dockerfile.