I am still using my own created channel list as described at your mentioned post:
Since I did the setup on my Synology DiskStation with DSM6 back then I just rechecked this with the latest DSM7 - the folder is still the same.
/volume1/@appstore/Plex Media Server/Resources/Tuner/Shared/scanners/dvbc/transponders
Since you are running Plex in a docker container I assume your path is slightly different, maybe something like /volume1/@docker/...
I guess you have to log in via SSH with root privileges since these folders are not visible in the GUI. Using a software like WinSCP makes this very easy.
Good luck! 
