Thanks!
Given that pointer, I figured out that I first needed to fix the permissions, then I still had an issue accessing the DB.
I ended up using portainer to make this stack:
--------
version: "3.7"
services:
   mariadb:
    image: lscr.io/linuxserver/mariadb:latest
    container_name: mariadb
    environment:
      - PUID=1000
      - PGID=1000
      - TZ=America/New_York
      - MYSQL_ROOT_PASSWORD=<somepassword>
      - MYSQL_DATABASE=piwigo #optional
      - MYSQL_USER=admin #optional
      - MYSQL_PASSWORD=<somepassword> #optional
      - REMOTE_SQL=http://URL1/your.sql,https://URL2/your.sql #optional
    volumes:
      - <wherever you put your docker configs>/mariadb/config:/config
    ports:
      - 3306:3306 
    restart: unless-stopped
 
   piwigo:
    image: lscr.io/linuxserver/piwigo:latest
    container_name: piwigo
    environment:
      - PUID=1000
      - PGID=1000
      - TZ=America/New_York
    volumes:
      - <wherever you put your docker configs>/config:/config
      - <wherever you put your docker configs>/piwigo/gallery:/gallery
    ports:
      - <incoming port>:80
    restart: unless-stopped

for the db server, all I needed was 'mariadb'