So, there might be a misconfiguration in my docker instance as I tried four of those cups dockers available on docker hub and they all produced the same error
That would suggest there is something residual from each install, if I do docker diff cupsd on that olbat/cups I get this;
A /var/cache/cups/help.index
A /var/cache/cups/job.cache
A /var/cache/cups/job.cache.O
A /var/cache/cups/ppds.dat
A /var/cache/cups/HP_ENVY_5640_series.strings
D /var/cache/cups/job.cache.N
C /var/log/cups/access_log
C /var/log/cups/error_log
A /etc/cups/printers.conf
A /etc/cups/printers.conf.O
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so I then do ls -l on /etc
-rw-r--r-- 1 root root 2981 Sep 21 16:24 adduser.conf
-rw-r--r-- 1 root root 44 Feb 11 08:05 adjtime
-rw-r--r-- 1 root root 51 Sep 21 16:26 aliases
-rw-r--r-- 1 root root 12288 Sep 21 16:26 aliases.db
drwxr-xr-x 2 root root 4096 Feb 24 11:37 alternatives
-rw-r--r-- 1 root root 401 May 19 2019 anacrontab
drwxr-xr-x 4 root root 4096 Feb 11 08:00 apache2
drwxr-xr-x 3 root root 4096 Feb 11 08:00 apm
drwxr-xr-x 5 root root 4096 Mar 2 18:57 apparmor.d
drwxr-xr-x 12 root root 4096 Feb 23 23:09 apt
drwxr-xr-x 3 root root 4096 Feb 11 08:35 avahi
-rw-r--r-- 1 root root 1994 Apr 18 2019 bash.bashrc
-rw-r--r-- 1 root root 45 Feb 11 2019 bash_completion
drwxr-xr-x 2 root root 4096 Mar 7 13:16 bash_completion.d
-rw-r--r-- 1 root root 367 Mar 2 2018 bindresvport.blacklist
drwxr-xr-x 2 root root 4096 Apr 27 2020 binfmt.d
drwxr-xr-x 3 root root 4096 Feb 11 08:00 ca-certificates
-rw-r--r-- 1 root root 6140 Feb 11 08:31 ca-certificates.conf
-rw-r--r-- 1 root root 5713 Sep 21 16:24 ca-certificates.conf.dpkg-old
drwxr-xr-x 2 root root 4096 Feb 11 08:00 calendar
drwxr-xr-x 2 root root 4096 Feb 11 08:15 chrony
drwxr-xr-x 3 root root 4096 Feb 11 08:00 collectd
drwxr-xr-x 2 root root 4096 Feb 11 08:02 console-setup
drwxr-xr-x 2 root root 4096 Mar 2 19:07 containerd
drwxr-xr-x 10 root root 4096 Feb 11 08:00 cron-apt
drwxr-xr-x 2 root root 4096 Mar 2 19:27 cron.d
drwxr-xr-x 2 root root 4096 Mar 7 13:16 cron.daily
drwxr-xr-x 2 root root 4096 Mar 4 10:11 cron.hourly
drwxr-xr-x 2 root root 4096 Feb 11 08:00 cron.monthly
-rw-r--r-- 1 root root 1042 Oct 11 2019 crontab
drwxr-xr-x 2 root root 4096 Mar 7 13:16 cron.weekly
drwxr-xr-x 4 root root 4096 Feb 11 08:00 dbus-1
-rw-r--r-- 1 root root 2969 Feb 26 2019 debconf.conf
-rw-r--r-- 1 root root 5 Jan 30 17:37 debian_version
drwxr-xr-x 3 root root 4096 Mar 7 13:16 default
-rw-r--r-- 1 root root 604 Jun 26 2016 deluser.conf
drwxr-xr-x 4 root root 4096 Feb 11 08:00 dhcp
drwxr-xr-x 4 root root 4096 Feb 25 14:11 dkms
drwxr-xr-x 2 root root 4096 Feb 11 09:54 docker
drwxr-xr-x 4 root root 4096 Feb 11 08:38 dpkg
-rw-r--r-- 1 root root 685 Jan 25 2020 e2scrub.conf
-rw-r--r-- 1 root root 0 Sep 21 16:28 environment
-rw-r--r-- 1 root root 389 Jun 24 2020 exports
drwxr-xr-x 2 root root 4096 Feb 11 08:24 folder2ram
drwxr-xr-x 4 root root 4096 Feb 11 08:00 fonts
-rw-r--r-- 1 root root 711 Feb 24 08:56 fstab
-rw-r--r-- 1 root root 922 Feb 24 08:56 fstab.bak
-rw-r--r-- 1 root root 132 Mar 10 2020 ftpusers
-rw-r--r-- 1 root root 280 Jun 20 2014 fuse.conf
-rw-r--r-- 1 root root 2584 Aug 1 2018 gai.conf
-rw-r--r-- 1 root root 949 Mar 4 10:11 group
-rw-r--r-- 1 root root 944 Mar 2 17:53 group-
drwxr-xr-x 2 root root 4096 Feb 19 15:53 grub.d
-rw-r----- 1 root shadow 806 Mar 4 10:11 gshadow
-rw-r----- 1 root shadow 801 Mar 2 17:53 gshadow-
drwxr-xr-x 3 root root 4096 Feb 11 08:00 gss
drwxr-xr-x 2 root root 4096 Feb 24 11:37 gtk-2.0
-rw-r--r-- 1 root root 5060 Oct 26 2018 hdparm.conf
-rw-r--r-- 1 root root 9 Aug 7 2006 host.conf
-rw-r--r-- 1 root root 4 Feb 11 08:38 hostid
-rw-r--r-- 1 root root 12 Mar 4 10:11 hostname
-rw-r--r-- 1 root root 407 Feb 11 08:26 hosts
-rw-r--r-- 1 root root 411 Sep 21 16:26 hosts.allow
-rw-r--r-- 1 root root 711 Sep 21 16:26 hosts.deny
-rw-r--r-- 1 root root 206 Jun 24 2020 idmapd.conf
drwxr-xr-x 3 root root 4096 Feb 11 08:00 ifplugd
drwxr-xr-x 2 root root 4096 Mar 7 13:16 init
drwxr-xr-x 2 root root 4096 Mar 7 13:16 init.d
drwxr-xr-x 5 root root 4096 Feb 11 08:00 initramfs-tools
-rw-r--r-- 1 root root 1748 May 5 2018 inputrc
drwxr-xr-x 2 root root 4096 Feb 11 08:00 insserv.conf.d
drwxr-xr-x 4 root root 4096 Feb 11 08:31 iproute2
drwxr-x--- 2 root root 4096 Feb 11 08:36 iptables
-rw-r--r-- 1 root root 594 Mar 7 20:28 issue
-rw-r--r-- 1 root root 20 Jul 10 2020 issue.net
drwxr-xr-x 8 root root 4096 Feb 25 14:11 kernel
-rw-r--r-- 1 root root 71 Feb 11 08:05 kernel-img.conf
drwxr-xr-x 2 root root 4096 Feb 23 09:47 ldap
-rw-r--r-- 1 root root 50344 Mar 2 19:27 ld.so.cache
-rw-r--r-- 1 root root 34 Mar 2 2018 ld.so.conf
drwxr-xr-x 2 root root 4096 Feb 11 08:00 ld.so.conf.d
-rw-r--r-- 1 root root 191 Apr 25 2019 libaudit.conf
drwxr-xr-x 2 root root 4096 Feb 11 08:00 libnl-3
-rw-r--r-- 1 root root 2995 May 1 2019 locale.alias
-rw-r--r-- 1 root root 9376 Feb 11 08:01 locale.gen
lrwxrwxrwx 1 root root 33 Feb 11 08:31 localtime -> /usr/share/zoneinfo/Europe/London
drwxr-xr-x 4 root root 4096 Feb 11 08:00 logcheck
-rw-r--r-- 1 root root 10477 Jul 27 2018 login.defs
-rw-r--r-- 1 root root 435 Aug 22 2018 logrotate.conf
drwxr-xr-x 2 root root 4096 Mar 7 13:16 logrotate.d
-rw-r--r-- 1 root root 73 Mar 25 2020 lsb-base-logging.sh
drwxr-xr-x 3 root root 4096 Feb 19 15:53 lvm
-rw-r--r-- 1 root root 33 Feb 11 08:12 machine-id
-rw-r--r-- 1 root root 111 Oct 22 2019 magic
-rw-r--r-- 1 root root 111 Oct 22 2019 magic.mime
-rw-r--r-- 1 root root 2514 Feb 24 11:49 mailcap
-rw-r--r-- 1 root root 449 Feb 9 2019 mailcap.order
-rw-r--r-- 1 root root 22 Sep 21 16:26 mailname
-rw-r--r-- 1 root root 125 Aug 8 2018 mail.rc
drwxr-xr-x 2 root root 4096 Feb 19 15:50 mdadm
-rw-r--r-- 1 root root 24512 Feb 9 2019 mime.types
-rw-r--r-- 1 root root 812 Feb 16 18:47 mke2fs.conf
drwxr-xr-x 2 root root 4096 Mar 7 13:16 modprobe.d
-rw-r--r-- 1 root root 195 Sep 21 16:24 modules
drwxr-xr-x 2 root root 4096 Mar 2 18:57 modules-load.d
drwxr-xr-x 6 root root 4096 Feb 11 08:00 monit
-rw-r--r-- 1 root root 286 Jul 10 2020 motd
lrwxrwxrwx 1 root root 19 Feb 11 08:12 mtab -> ../proc/self/mounts
-rw-r--r-- 1 root root 9278 Jun 12 2019 nanorc
-rw-r--r-- 1 root root 767 Dec 11 2018 netconfig
drwxr-xr-x 2 root root 4096 Feb 11 08:26 netplan
drwxr-xr-x 7 root root 4096 Feb 11 08:26 network
drwxr-xr-x 3 root root 4096 Feb 11 08:00 NetworkManager
-rw-r--r-- 1 root root 60 Sep 21 16:24 networks
drwxr-xr-x 9 root root 4096 Feb 11 08:00 nginx
-rw-r--r-- 1 root root 537 Sep 21 16:27 nsswitch.conf
drwxr-xr-x 2 root root 4096 Mar 7 13:16 openmediavault
drwxr-xr-x 2 root root 4096 Sep 21 16:23 opt
lrwxrwxrwx 1 root root 21 Jan 30 17:37 os-release -> ../usr/lib/os-release
-rw-r--r-- 1 root root 552 Feb 14 2019 pam.conf
drwxr-xr-x 2 root root 4096 Mar 7 13:16 pam.d
-rw-r--r-- 1 root root 1927 Mar 4 10:11 passwd
-rw-r--r-- 1 root root 1889 Feb 11 08:14 passwd-
drwxr-xr-x 2 root root 4096 Feb 11 08:00 pcmcia
drwxr-xr-x 4 root root 4096 Feb 11 08:00 perl
drwxr-xr-x 3 root root 4096 Feb 11 08:00 php
drwxr-xr-x 5 root root 4096 Mar 4 10:11 postfix
drwxr-xr-x 4 root root 4096 Feb 11 08:00 ppp
-rw-r--r-- 1 root root 767 Mar 4 2016 profile
drwxr-xr-x 2 root root 4096 Feb 11 08:23 profile.d
drwxr-xr-x 3 root root 4096 Feb 11 08:36 proftpd
-rw-r--r-- 1 root root 2932 Feb 10 2019 protocols
drwxr-xr-x 3 root root 4096 Feb 24 11:37 pulse
drwxr-xr-x 2 root root 4096 Feb 11 08:00 python
drwxr-xr-x 2 root root 4096 Feb 11 08:00 python2.7
drwxr-xr-x 2 root root 4096 Feb 11 08:00 python3
drwxr-xr-x 2 root root 4096 Feb 11 08:00 python3.7
-rw-r--r-- 1 root root 220 Sep 11 2019 quotagrpadmins
-rw-r--r-- 1 root root 526 Feb 1 2018 quotatab
drwxr-xr-x 2 root root 4096 Mar 2 19:07 rc0.d
drwxr-xr-x 2 root root 4096 Mar 2 19:07 rc1.d
drwxr-xr-x 2 root root 4096 Mar 4 10:10 rc2.d
drwxr-xr-x 2 root root 4096 Mar 4 10:10 rc3.d
drwxr-xr-x 2 root root 4096 Mar 4 10:10 rc4.d
drwxr-xr-x 2 root root 4096 Mar 4 10:10 rc5.d
drwxr-xr-x 2 root root 4096 Mar 2 19:07 rc6.d
drwxr-xr-x 2 root root 4096 Feb 24 11:37 rcS.d
-rw-r--r-- 1 root root 1889 Mar 6 2019 request-key.conf
drwxr-xr-x 2 root root 4096 Feb 11 08:00 request-key.d
lrwxrwxrwx 1 root root 32 Feb 11 08:26 resolv.conf -> /run/systemd/resolve/resolv.conf
drwxr-xr-x 3 root root 4096 Feb 11 08:00 resolvconf
lrwxrwxrwx 1 root root 13 Feb 11 08:01 rmt -> /usr/sbin/rmt
-rw-r--r-- 1 root root 887 Feb 10 2019 rpc
-rw-r--r-- 1 root root 1988 Feb 26 2019 rsyslog.conf
drwxr-xr-x 2 root root 4096 Mar 7 13:16 rsyslog.d
drwxr-xr-x 5 root root 4096 Feb 11 08:32 salt
drwxr-xr-x 3 root root 4096 Mar 4 10:10 samba
-rw-r--r-- 1 root root 4141 Jul 27 2018 securetty
drwxr-xr-x 4 root root 4096 Feb 11 08:00 security
drwxr-xr-x 2 root root 4096 Feb 11 08:00 selinux
-rw-r--r-- 1 root root 10593 Dec 19 2018 sensors3.conf
drwxr-xr-x 2 root root 4096 Feb 24 11:49 sensors.d
-rw-r--r-- 1 root root 18774 Feb 10 2019 services
-rw-r----- 1 root shadow 1149 Mar 4 10:11 shadow
-rw-r----- 1 root shadow 1089 Feb 11 08:14 shadow-
-rw-r--r-- 1 root root 116 Sep 21 16:24 shells
drwxr-xr-x 2 root root 4096 Feb 11 08:00 skel
-rw-r--r-- 1 root root 7042 Jan 15 2020 smartd.conf
drwxr-xr-x 4 root root 4096 Feb 11 08:00 smartmontools
-rw-r--r-- 1 root root 313 Feb 19 16:45 snapraid.conf
-rw-r--r-- 1 root root 273 Feb 19 16:45 snapraid-diff.conf
drwxr-xr-x 2 root root 4096 Feb 11 08:01 ssh
drwxr-xr-x 4 root root 4096 Feb 19 15:53 ssl
-rw-r--r-- 1 root root 19 Mar 4 10:11 subgid
-rw-r--r-- 1 root root 0 Sep 21 16:24 subgid-
-rw-r--r-- 1 root root 19 Mar 4 10:11 subuid
-rw-r--r-- 1 root root 0 Sep 21 16:24 subuid-
-r--r----- 1 root root 669 Feb 2 2020 sudoers
drwxr-xr-x 2 root root 4096 Mar 2 19:27 sudoers.d
-rw-r--r-- 1 root root 2351 May 31 2018 sysctl.conf
drwxr-xr-x 2 root root 4096 Mar 7 13:16 sysctl.d
drwxr-xr-x 5 root root 4096 Feb 11 08:30 systemd
drwxr-xr-x 2 root root 4096 Feb 11 08:00 terminfo
-rw-r--r-- 1 root root 14 Feb 11 08:31 timezone
drwxr-xr-x 2 root root 4096 Apr 27 2020 tmpfiles.d
-rw-r--r-- 1 root root 1260 Dec 14 2018 ucf.conf
drwxr-xr-x 4 root root 4096 Feb 11 08:31 udev
drwxr-xr-x 3 root root 4096 Feb 11 08:00 ufw
drwxr-xr-x 2 root root 4096 Feb 11 08:30 update-motd.d
-rw-r--r-- 1 root root 51 Jan 20 2019 vdpau_wrapper.cfg
drwxr-xr-x 2 root root 4096 Feb 11 08:00 vim
-rw-r--r-- 1 root root 314 Feb 1 2018 warnquota.conf
-rw-r--r-- 1 root root 312 Sep 21 16:27 watchdog.conf
-rw-r--r-- 1 root root 4942 Apr 5 2019 wgetrc
drwxr-xr-x 2 root root 4096 Feb 11 08:00 wpa_supplicant
drwxr-xr-x 6 root root 4096 Feb 24 11:37 X11
-rw-r--r-- 1 root root 642 Mar 1 2019 xattr.conf
drwxr-xr-x 3 root root 4096 Feb 11 08:00 xdg
drwxr-xr-x 4 root root 4096 Mar 2 19:27 zfs
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based on the above output C at the beginning of /etc/cups must refernce container, so it's not written to the system.
If you had access to a config file in /etc/cups then that must have been residual.
I'm an IT professional (cyber security currently with 10 years of sys admin) so you can trust that I have a rough idea what I'm doing
TBH more tech nerds on here the better, me I try and work on a process of elimination to try and help, I also have VM with omv5 to try out on which is where I have this docker deployed