Bug#919659: live-build: building in docker fails with mounting /proc unmount /sys

[Leszek Lesner <leszek@zevenos.com>]

Package: live-build
Version: 1:20180925
Severity: important

Dear Maintainer,

live-build fails building images/ISOs in docker. 
The only errors and warnings I found in the logs were those:
> I: Extracting zlib1g...
> W: Failure trying to run: chroot "/build/c1-live-build-core/chroot" mount -t
> proc proc /proc W: See
> /build/c1-live-build-core/chroot/debootstrap/debootstrap.log for details I:
> Installing core packages...
> 
> P: Begin unmounting /sys...
> umount: chroot/sys: umount failed: No such file or directory.
> P: Begin unmounting filesystems...

Sadly I cannot access the debootstrap.log file as it is non existing. 
The same live-build configs work fine building in a stretch docker image. 

-- System Information:
Debian Release: buster/sid
  APT prefers testing
  APT policy: (500, 'testing')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 4.18.0-3-amd64 (SMP w/3 CPU cores)
Locale: LANG=en_US.UTF-8, LC_CTYPE=en_US.UTF-8 (charmap=UTF-8),
LANGUAGE= (charmap=UTF-8)
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled


-- 
ZevenOS / Neptune Team
https://neptuneos.com
Leszek Lesner <leszek@zevenos.com>