did you change your data-root (defaults to /var/lib/docker) folder to a filesystem supported as backing filesystem (ext4/xfs) and restarted the daemon?
see:
update:
I missed the Synology part: no overlay2 on Synology for time beeing - even though it seems to be supported on the 1819+, the support is broken. On most models, overlay2 is not even an option. Additionaly: why do use still such an old version of Synology’s Docker package? And last but not least the daemon.json is located in a different path and a different name on Synology’s Docker: /var/packages/Docker/etc/dockerd.json
You best bet is to raise a support ticket @Synology. Their docker redistribution is modified and maintained by them.
Thank you for the informations, the problem fixed.
i updated the DSM, put a extra drive into the nas and set up with ext4 then install docker on it, after that edit the correct file what you write down (/var/packages/Docker/etc/dockerd.json)
May i ask which Synology model and DSM version it is? So far I haven’t encountered a single model that worked flawless with overlay2. I know that they all work like charme with btrfs and aufs, but a model with actualy not broken overlay2 support is new to me.