Xdebug ignoring XDEBUG_CONFIG set in docker-compose.yml – Debian

I have a docker-compose file that has the XDEBUG_CONFIG environment variable configured, but Xdebug is ignoring it. When I output xdebug_info(); it says "no value" for xdebug.idekey. XDEBUG_MODE is being read correctly though.

If I set the idekey in my .ini file, it works.

PHP 7.4
Xdebug 3.1.2

services:
    app:
        build:
            context: ./docker
            args:
                WWWUSER: ${WWWUSER:-1000}
                WWWGROUP: ${WWWGROUP:-1000}
        environment:
            PHP_IDE_CONFIG: "serverName=${PHP_IDE_CONFIG:-localhost}"
            XDEBUG_CONFIG: "idekey=VSCODE"
            XDEBUG_MODE: '${XDEBUG_MODE:-debug,develop}'
        volumes:
            - .:/var/www/html
            - ./docker/php/local.ini:/usr/local/etc/php/conf.d/local.ini
            - ./docker/php/xdebug.ini:/usr/local/etc/php/conf.d/xdebug.ini
            - ./docker/php/php.ini-development:/usr/local/etc/php/php.ini
        depends_on:
            - mysql
            - redis
            - nginx

Here’s my full docker-compose.yml

version: '3.9'

services:
    app:
        build:
            context: ./docker
            args:
                WWWUSER: ${WWWUSER:-1000}
                WWWGROUP: ${WWWGROUP:-1000}
        environment:
            PHP_IDE_CONFIG: "serverName=${PHP_IDE_CONFIG:-localhost}"
            XDEBUG_CONFIG: "idekey=VSCODE"
            XDEBUG_MODE: '${XDEBUG_MODE:-debug,develop}'
        volumes:
            - .:/var/www/html
            - ./docker/php/local.ini:/usr/local/etc/php/conf.d/local.ini
            - ./docker/php/xdebug.ini:/usr/local/etc/php/conf.d/xdebug.ini
            - ./docker/php/php.ini-development:/usr/local/etc/php/php.ini
        depends_on:
            - mysql
            - redis
            - nginx

    mysql:
        image: mysql:8.0
        environment:
            MYSQL_DATABASE: '${DB_DATABASE}'
            MYSQL_ROOT_PASSWORD: '${DB_PASSWORD}'
            MYSQL_USER: '${DB_USERNAME}'
            MYSQL_PASSWORD: '${DB_PASSWORD}'
            MYSQL_ALLOW_EMPTY_PASSWORD: 'yes'
        volumes:
            - mysql-data:/var/lib/mysql
            - ./docker/mysql:/docker-entrypoint-initdb.d
        ports:
            - ${FORWARD_DB_PORT:-3306}:3306
        healthcheck:
            test: ["CMD", "mysqladmin", "ping"]

    redis:
        image: redis:alpine
        volumes:
            - redis-data:/data
        ports:
            - ${FORWARD_REDIS_PORT:-6379}:6379
        healthcheck:
            test: ["CMD", "redis-cli", "ping"]

    nginx:
        image: nginx:alpine
        volumes:
            - .:/var/www/html
            - ./docker/nginx/conf.d:/etc/nginx/conf.d
        ports:
            - ${NGINX_PORT:-80}:80

volumes:
    mysql-data:
        driver: local
    redis-data:
        driver: local

And my Dockerfile

FROM php:7.4-fpm

# Install extensions
ADD https://github.com/mlocati/docker-php-extension-installer/releases/latest/download/install-php-extensions /usr/local/bin/
RUN chmod +x /usr/local/bin/install-php-extensions && 
    install-php-extensions 
    gd 
    xdebug 
    memcached 
    imagick 
    @composer 
    pdo_mysql 
    exif 
    pcntl 
    zip

WORKDIR /var/www/html

# Install dependencies
RUN apt-get update && apt-get install -y 
    build-essential 
    libpng-dev 
    libjpeg62-turbo-dev 
    libfreetype6-dev 
    locales 
    libzip-dev 
    zip 
    unzip 
    jpegoptim optipng pngquant gifsicle 
    vim 
    git 
    curl

RUN curl -sL https://deb.nodesource.com/setup_16.x | bash - 
    && apt-get install -y nodejs 
    && curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | apt-key add - 
    && echo "deb https://dl.yarnpkg.com/debian/ stable main" > /etc/apt/sources.list.d/yarn.list 
    && apt-get update 
    && apt-get install -y yarn

# Clean
RUN apt-get -y autoremove 
    && apt-get clean

ARG WWWUSER
ARG WWWGROUP

RUN groupadd --force -g $WWWGROUP theusergroup 
    && useradd -ms /bin/bash --no-user-group -g $WWWGROUP -u $WWWUSER theuser

USER $WWWUSER:$WWWGROUP

# Expose port 9000 and start php-fpm server
EXPOSE 9000

CMD ["php-fpm"]