I have been struggling with this for weeks now.
Problem: every time I reboot my OMV server plex doesn't work. All Network Shares and NAS shares are unavailable at boot so the service would have to be restarted from docker after a reboot to fix the issue.
First I delayed the docker start-up by 30 seconds.
I added the following line to /etc/systemd/system/multi-user.target.wants/docker.service
It should look something like this.
# the default is not to use systemd for cgroups because the delegate issues still
# exists and systemd currently does not support the cgroup feature set required
# for containers run by docker
ExecStart=/usr/bin/dockerd -H fd:// --containerd=/run/containerd/containerd.sock
ExecReload=/bin/kill -s HUP $MAINPID
This last step isn't needed but if you have any MNT's that take a while to load and 30 seconds isn't fixing it for you add "mnt-nas.mount" to the following areas in the same file.
Description=Docker Application Container Engine
After=network-online.target firewalld.service containerd.service mnt-nas.mount
Requires=docker.socket containerd.service mnt-nas.mount
This should fix it. At least it fixed it for me.