Please take a closer look at the PGID and PUID variables of the linuxserverio image (refer to "User / Group Identifiers" in the instructions). You want these to reflect your syno's dockeruser IDs to avoid using su rights. You can get them via SSH. For my DS its PUID 1031 and PGID 100.
Perhaps consider using a docker image liike linuxserver/ubooquity:latest for a really effortless installation and maintenance...
Usually you would not need to use this repository at all, since the docker app is part of the official synology app suite and can be installed and managed quite easily from within the DSM package manager.
Aside from that, I think the source of your issues is your containers configuration. Make sure you have mapped the required folders correctly, applied the necessary access rights to these folders so that docker can actually access them and provide the recommended environment variables and port allocations. The linuxserver/ubooquity docker actually makes a very good job in providing the relevant instructions on the page I linked.
Try pulling the linuxserver/ubooquity directly from within synology's docker app.
The directory you listed contains synology's docker versions. You should not need these normally. You can manage everything from within the DSM.
Perhaps you cold try the most current version of Ubooquity? I am running the linuxserver docker without any problems on my 1517+.
Sorry, but without something more substantial then "I tried all the setup tutorials" I find it rather difficult to give any pointers, since I don't know what you did at all.
I'd suggest you look over a few other posts with similar topics and then ask a little mor specific.
I never changed any network settings for docker in my setup, so I dont really think that has anything to do with your problems. Docker itself should take care of mapping the container ports from its bridge subnet to the ports specified for your host (=NAS).
Mount point should be "/comics" (lowercase). You also need to provide a folder for the configuration filses (settings and database) and set it to mount point "/config". Make sure this directory is writable for Docker.
Then check if the container and Ubooquity is actually starting by looking at the output in the Terminal tab of the running containers details window. Any errors should also be in the Logging tab.
Running fine wih +104000 books.
You got the Server and Container sides of the volume settings mixed up. Left side is the folder location on your NAS, and right side the path inside the container where the folder is mounted (the idea is, that the data locations can be outside the container, but the sytem running inside the container can access it like it were inside its own file structure). So just switch it (provided your comics are on your NAS within /volume1/Comics).
Similar with the Ports. Right side are the ports of the Containers system, left where they are mapped to on your NAS so you can access them. You can either look on the running containers details window where they are mapped to automatically if you leave your settings on Auto (but know that these assignments are dynamic an can change after restaring the container) or you can assign the ports yourself (recommended, usually you can just use the same ones like within the container).
On a sidenote, for safety reasons I would not recommend acessing the data-directories as admin, the Dockeruser ("docker" on my DS) is a much safer way. Naturally you have to make sure the Dockeruser has access rights to your Comic folder.
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