Docker build is failing on Ubuntu

I am trying to build a Docker image of Volumio from Ubuntu, using the instructions from Github: volumio.github.io/docs/Developm … ages.html#

It somewhat works - there a lots of warnings and several errors, such as these:
Err:1 archive.volumio.org/raspbian jessie InRelease
The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 9165938D90FDDD2E
Reading package lists… Done
W: GPG error: archive.volumio.org/raspbian jessie InRelease: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 9165938D90FDDD2E
E: The repository ‘http://archive.volumio.org/raspbian jessie InRelease’ is not signed.
N: Updating from such a repository can’t be done securely, and is therefore disabled by default.

cp: cannot create regular file ‘build/arm/root/etc/dhcp/dhclient.conf’: No such file or directory
cp: cannot create regular file ‘build/arm/root/etc/dhcp/dhcpd.conf’: No such file or directory
cp: cannot create regular file ‘build/arm/root/etc/samba/smb.conf’: No such file or directory
.
.
.

But in the end it does build an image file (which I have not yet tested):
Writing Raspberry Pi Image File
Creating Image Bed
Image file: Volumio2.0-2018-12-31-pi.img
2800+0 records in
2800+0 records out
2936012800 bytes (2.9 GB, 2.7 GiB) copied, 29.297 s, 100 MB/s
Warning: The resulting partition is not properly aligned for best performance.
Warning: The resulting partition is not properly aligned for best performance.
Warning: The resulting partition is not properly aligned for best performance.
Model: Loopback device (loopback)
Disk /dev/loop34: 2936MB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Disk Flags:

Number Start End Size Type File system Flags
1 512B 64.0MB 64.0MB primary boot, lba
2 64.0MB 2500MB 2436MB primary
3 2500MB 2800MB 300MB primary

But the build script never exits (at least, I let it run for a couple of hours and it had not exited). I finally killed it, but there is no sign of a Docker image. Where should I find the Docker image? Any ideas why the build script never exits?