Since there doesn't seem to be an all-in-one tutorial on the web.
Linking shares to container
Step 1 - permitted characters:
Piwigo doesn't like spaces and non-englisch characters. Your filenames are probably filled with them. If not, skip.
Start Piwigo and go to plugins and install "Local files editor".
Open this editor and in "local configuration" find
$conf['sync_chars_regex'] = '/^[a-zA-Z0-9-_.]+$/';
You can add or remove chracter to fit your needs. As long as you mind the UTF-8 encoding.
Latin languages $conf['sync_chars_regex'] = '/^[(a-zA-Z0-9-_.+&áéíóúàãõ@ )]+$/';
Germans $conf['sync_chars_regex'] ='/^[(a-zA-Z0-9-_.ÄäÖöÜüß@ )]+$/';
Etc
Keep your filenames civil and don't anger the html guys. & # ' " can be troublesome.
Special characters are also dependant on your os.
You'll find out in step 3.
Step 2 - make the link in unraid:
Add a path in the docker config.
Step 3 - sync the files/folders inside Piwigo:
Go to admin > tools > synchronize
This part is self explanatory. Don't forget to turn of simulation.