The pdo_mysql extension is not detected on Wordpress Docker container https://stackoverflow.com/questions/57446636/the-pdo-mysql-extension-is-not-detected-on-wordpress-docker-container

docker-compose.yml

version: '3'

services:
   db:
     container_name: ${CONTAINER_DB_NAME}
     image: mariadb:latest
     restart: unless-stopped
     volumes:
        - ${DB_PATH}:/var/lib/mysql
     environment:
       MYSQL_ROOT_PASSWORD: ${MYSQL_ROOT_PASSWORD}
       MYSQL_DATABASE: ${MYSQL_DATABASE}
       MYSQL_USER: ${MYSQL_USER}
       MYSQL_PASSWORD: ${MYSQL_PASSWORD}

   wordpress:
     depends_on:
       - db
     container_name: ${CONTAINER_WP_NAME}
     # image: wordpress:latest
     build:
       context: .
       dockerfile: Dockerfile-WordPressPDO
     restart: unless-stopped
     volumes:
       - ${WP_CORE}:/var/www/html
       - ${WP_CONTENT}:/var/www/html/wp-content
       - ./conf.d/php.ini:/usr/local/etc/php/conf.d/php.ini
     environment:
       WORDPRESS_DB_HOST: ${CONTAINER_DB_NAME}:3306
       WORDPRESS_DB_NAME: ${MYSQL_DATABASE}
       WORDPRESS_DB_USER: ${MYSQL_USER}
       WORDPRESS_DB_PASSWORD: ${MYSQL_PASSWORD}
       WORDPRESS_TABLE_PREFIX: ${WORDPRESS_TABLE_PREFIX}
       VIRTUAL_HOST: ${DOMAINS}
       LETSENCRYPT_HOST: ${DOMAINS}
       LETSENCRYPT_EMAIL: ${LETSENCRYPT_EMAIL}
     logging:
       options:
         max-size: ${LOGGING_OPTIONS_MAX_SIZE:-200k}

#   wpcli:
#     image: tatemz/wp-cli
#     volumes:
#       - ${WP_CORE}:/var/www/html
#       - ${WP_CONTENT}:/var/www/html/wp-content
#     depends_on:
#       - db
#     entrypoint: wp

networks:
    default:
       external:
         name: ${NETWORK}

Dockerfile-WordPressPDO

FROM php:5.6-apache

RUN a2enmod rewrite

# install the PHP extensions we need
RUN apt-get update && apt-get install -y libpng12-dev libjpeg-dev && rm -rf /var/lib/apt/lists/* \
	&& docker-php-ext-configure gd --with-png-dir=/usr --with-jpeg-dir=/usr \
	&& docker-php-ext-install gd
RUN docker-php-ext-install mysqli
RUN docker-php-ext-install pdo php_pdo

VOLUME /var/www/html

ENV WORDPRESS_VERSION 4.2.1
ENV WORDPRESS_UPSTREAM_VERSION 4.2.1
ENV WORDPRESS_SHA1 c93a39be9911591b19a94743014be3585df0512f

# upstream tarballs include ./wordpress/ so this gives us /usr/src/wordpress
RUN curl -o wordpress.tar.gz -SL https://wordpress.org/wordpress-${WORDPRESS_UPSTREAM_VERSION}.tar.gz \
	&& echo "$WORDPRESS_SHA1 *wordpress.tar.gz" | sha1sum -c - \
	&& tar -xzf wordpress.tar.gz -C /usr/src/ \
	&& rm wordpress.tar.gz \
	&& chown -R www-data:www-data /usr/src/wordpress

COPY docker-entrypoint.sh /entrypoint.sh

# grr, ENTRYPOINT resets CMD now
ENTRYPOINT ["/entrypoint.sh"]
CMD ["apache2-foreground"]