Docker container working as a mail server runs out of space

I will try to describe the issue with as many details as possible.

I have a Digital Ocean server 1GB RAM, 25GB SSD with Ubuntu 16.04. I have a docker container (docker version is: Docker version 18.03.0-ce, build 0520e24).

The docker container contains postfix, dovecot, mysql, roundcube and a few other services.

Everything works fine but when start to receive or send emails the free space diminishes and I end up with no free space.

Before that happens if I run “df -h” I get:

root@mail:~# df -h
Filesystem      Size  Used Avail Use% Mounted on
udev            488M     0  488M   0% /dev
tmpfs           100M  9.3M   90M  10% /run
/dev/vda1        25G   11G   14G  46% /
tmpfs           497M     0  497M   0% /dev/shm
tmpfs           5.0M     0  5.0M   0% /run/lock
tmpfs           497M     0  497M   0% /sys/fs/cgroup
/dev/vda15      105M  3.4M  102M   4% /boot/efi
overlay          25G   11G   14G  46% /var/lib/docker/overlay2/17795ba052726099d7ca962eb9756c8e5354483e388c3e326781249954172746/merged
shm              64M     0   64M   0% /var/lib/docker/containers/559088336f8c6948d6940714b7e97aea765bc98ee6f237afe39406b2f3638e67/mounts/shm

When I run out of space “/dev/vda1” says “Used space 25GB” and also “overlay” says “Used 25GB”.

When I access the docker container:

docker attach SOME_ID

and I try to find directories and files of significant size or directories and files generated recently…supposing that in this way I will find the source of the issue…I can’t find anything.

This is quite confusing because according to “df -h” there is no free space and the issue is in the docker container but when I try to find suspicious files I can’t find any.

And also, if I reboot the whole machine (Turn off -> Turn on) the used space is emptied and the mail server gets back to normal. In a few days the issue happens again. The server runs out of space and I have to reboot it again.

May I please ask you for advice how to trace the source of the issue?