デフォルトだと10Mしかアップロードできないので制限を緩和します🐱✨

docker-compose.yml

docker
│
│-phpmyadmin
│  └phpmyadmin-misc.ini ←●追加
│

phpmyadmin-misc.ini

allow_url_fopen = Off
max_execution_time = 600
memory_limit = 64M
post_max_size = 64M
upload_max_filesize = 64M

docker-compose.yml

version: '3'

services: 

    db:
        image: mysql:5.6
        container_name: db-host
        environment: 
            MYSQL_ROOT_PASSWORD: root
            MYSQL_DATABASE: sampledb
            MYSQL_USER: homestead
            MYSQL_PASSWORD: secret
            TZ: 'Azia/Tokyo'
        command: mysqld --character-set-server=utf8mb4 --collation-server=utf8mb4_unicode_ci
        volumes: 
            - ./docker/db/data:/var/lib/mysql
            - ./docker/db/my.cnf:/etc/mysql/conf.d/my.cnf
            - ./docker/db/sql:/docker-entrypoint-initdb.d
        ports:
            - 13306:3306

    phpmyadmin:
        container_name: phpmyadmin
        image: phpmyadmin/phpmyadmin
        environment: 
            - PMA_ARBITRARY=1
            - PMA_HOSTS=db-host
            - PMA_USER=root
            - PMA_PASSWORD=root
        volumes: 
            - ./docker/phpmyadmin/phpmyadmin-misc.ini:/usr/local/etc/php/conf.d/phpmyadmin-misc.ini ←●追加
        ports:
            - 18080:80

docker再起動

$ docker-compose up -d