message: Fix Docker related issues for gitian-build.py
  The Docker does not depend on apt-cacher-ng package.
  Do not try to install the Docker if docker.service is detected on the
  system (e.g., the Docker was installed manually).
  Also small style corrections were applied.
diff: @@ -7,20 +7,20 @@ import sys
  def setup():
  global args, workdir
  - programs = ['ruby', 'git', 'apt-cacher-ng', 'make', 'wget']
  + programs = ['ruby', 'git', 'make', 'wget']
  if args.kvm:
  - programs += ['python-vm-builder', 'qemu-kvm', 'qemu-utils']
  - elif args.docker:
  + programs += ['apt-cacher-ng', 'python-vm-builder', 'qemu-kvm', 'qemu-utils']
  + elif args.docker and not os.path.isfile('/lib/systemd/system/docker.service'):
  dockers = ['docker.io', 'docker-ce']
  for i in dockers:
  return_code = subprocess.call(['sudo', 'apt-get', 'install', '-qq', i])
  if return_code == 0:
  break
  if return_code != 0:
  - print('Cannot find any way to install docker', file=sys.stderr)
  - exit(1)
  + print('Cannot find any way to install Docker.', file=sys.stderr)
  + sys.exit(1)
  else:
  - programs += ['lxc', 'debootstrap']
  + programs += ['apt-cacher-ng', 'lxc', 'debootstrap']
  subprocess.check_call(['sudo', 'apt-get', 'install', '-qq'] + programs)
  if not os.path.isdir('gitian.sigs'):
  subprocess.check_call(['git', 'clone', 'https://github.com/bitcoin-core/gitian.sigs.git'])
  @@ -41,7 +41,7 @@ def setup():
  if args.is_bionic and not args.kvm and not args.docker:
  subprocess.check_call(['sudo', 'sed', '-i', 's/lxcbr0/br0/', '/etc/default/lxc-net'])
  print('Reboot is required')
  - exit(0)
  + sys.exit(0)
  def build():
  global args, workdir
  @@ -193,11 +193,11 @@ def main():
  if args.signer == '':
  print(script_name+': Missing signer.')
  print('Try '+script_name+' --help for more information')
  - exit(1)
  + sys.exit(1)
  if args.version == '':
  print(script_name+': Missing version.')
  print('Try '+script_name+' --help for more information')
  - exit(1)
  + sys.exit(1)
  # Add leading 'v' for tags
  if args.commit and args.pull:
  