I downloaded your sample then went and changed user/paswords, but it looks like the user "app" is somewhere hard-coded as I get these errors from the stack:

r3-db   | 2023-04-03 12:42:16.456 UTC [72] FATAL:  password authentication failed for user "app"                                                       
r3-db   | 2023-04-03 12:42:16.456 UTC [72] DETAIL:  Role "app" does not exist.

btw. here is my changed compose file and the .env file

version: '3.3'                                                                                                                                         
# https://github.com/r3-team/r3                                                                                                                        
                                                                                                                                                       
services:                                                                                                                                              
    r3-db:                                                                                                                                             
        container_name: r3-db                                                                                                                          
        hostname: r3-db                                                                                                                                
        image: postgres:latest                                                                                                                         
        environment:                                                                                                                                   
            POSTGRES_PASSWORD: $POSTGRES_PASSWORD                                                                                                      
            POSTGRES_USER: r3                                                                                                                          
        restart: "no"                                                                                                                                  
        volumes:                                                                                                                                       
            - ./data/db:/var/lib/postgresql/data                                                                                                       
        networks:                                                                                                                                      
          - r3                                                                                                                                         
    r3:                                                                                                                                                
        build:                                                                                                                                         
            context: https://github.com/r3-team/r3_docker.git#main                                                                                     
            args:                                                                                                                                      
                r3_db_host: r3-db                                                                                                                      
                r3_os_arch: x64                                                                                                                        
                r3_version: ${R3_VERSION:-3.3}                                                                                                         
                im_policy: /etc/ImageMagick-6/policy.xml                                                                                               
        image: "r3/${R3_VERSION:-3.3}"                                                                                                                 
        container_name: r3-app                                                                                                                         
        hostname: r3-app                                                                                                                               
        depends_on:                                                                                                                                    
            - r3-db                                                                                                                                    
        ports:                                                                                                                                         
            - "14000:80"                                                                                                                               
        restart: "no"                                                                                                                                  
        volumes:                                                                                                                                       
            - ./data/app:/opt/r3/data                                                                                                                  
        networks:                                                                                                                                      
          - r3                                                                                                                                         
                                                                                                                                                       
networks:                                                                                                                                              
  r3:                                                                                                                                                  
    external: true                                                                                                                                     
    name: r3

POSTGRES_PASSWORD=supersecretpw
R3_VERSION=3.3