Backup doesn't work with disabled IPv6

[RenHoekNL]

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Description

Mailcow backup tries to access '/proc/sys/net/ipv6/conf/all/disable_ipv6' on a Linux server that has IPv6 disabled, leading to an error.

Logs:

I've disabled IPv6 on my Linux server. I followed the guide (https://docs.mailcow.email/post_installation/firststeps-disable_ipv6/) to fix up Mailcow to run solely IPv4. This all works.

However when I want to do a backup:
MAILCOW_BACKUP_LOCATION=/backup/mailcow THREADS=4 /opt/mailcow-dockerized/helper-scripts/backup_and_restore.sh backup all

I get the following error:
Using SQL image mariadb:10.5, starting...
docker: Error response from daemon: failed to create task for container: failed to create shim task: OCI runtime create failed: runc create failed: unable to start container process: error during container init: open /proc/sys/net/ipv6/conf/all/disable_ipv6: no such file or directory: unknown.
ERRO[0000] error waiting for container: context canceled

Steps to reproduce:

1) Disable IPv6 via the ipv6.disable=1 boot flag
2) Configure Mailcow to run without IPv6 with the guide
3) Try to make a backup

Which branch are you using?

master

Which architecture are you using?

x86

Operating System:

Ubuntu 22.04.4 LTS

Server/VM specifications:

not applicable

Is Apparmor, SELinux or similar active?

not applicable

Virtualization technology:

not applicable

Docker version:

not applicable

docker-compose version or docker compose version:

not applicable

mailcow version:

latest

Reverse proxy:

not applicable

Logs of git diff:

not applicable

Logs of iptables -L -vn:

not applicable

Logs of ip6tables -L -vn:

not applicable

Logs of iptables -L -vn -t nat:

not applicable

Logs of ip6tables -L -vn -t nat:

not applicable

DNS check:

not applicable