RushLana, ---  THANK YOU !!!

I followed your example and I can use multiple locations !!!   

NOW,,,, for the elephant in the room question.......

Why with basic install can piwigo find the galleries folder and the sub-folders and pictures inside the sub-folders ???
1st  location --    /home/anne/Piwigo/piwigo-data/galleries/folder1/pictures-ABC.jpg

You have ,,,
    volumes:
      - ./piwigo-data/piwigo:/var/www/html/piwigo/
      - ./piwigo-data/scripts:/usr/local/bin/scripts/

AND ------ piwigo finds and displays the data in the galleries folder.....

---  SO ---  ,,,,,   why when I add a 2nd location    - /home/anne/Piwigo-media/:/var/www/html/piwigo/galleries
does piwigo only find the newly added 2nd location......  and deletes the data from the 1st location ????

----BUT, --  ,,,,,,  if I add a 3rd location,

    volumes:
      - ./piwigo-data/piwigo:/var/www/html/piwigo/          1st. the Galleries Folder, sub-folders and pictures in sub-folders
      - ./piwigo-data/scripts:/usr/local/bin/scripts/

      - /home/anne/Piwigo-media/:/var/www/html/piwigo/galleries  ADD 2nd,  ONLY this location shows,, deletes data from 1st

      - /home/anne/Pictures/:/var/www/html/piwigo/galleries/Pictures/  ADD 3rd, now  2 and 3 display,  1st still does not

Having BOTH the 2nd & 3rd locations show up,,,,  is GREAT !!   

BUT------

the ORIGINAL  location   /home/anne/Piwigo/piwigo-data/galleries/folder1/pictures-ABC.jpg
still does NOT show up......

I can certainly work around the 1st original location not showing by just adding another location, and move the data from location 1,  to the new location.

I hope all this description makes sense to you.....  All this is because I really would like to know and understand  WHY the original location 1,  works great  IF  that location is the only location,,,,  does not work when other locations are added....

THANK YOU
for all your guidance in getting this to work...
and I understand if you do not have time to "school" this old lady in docker stuff.... :-)
Anne