How to install pdo driver in php docker image?

I am using as a base the php docker container with the tag:

php:5.6-apache

I linked it with a basic mysql:5.6 image which I can reach at the host mysql. I created a DB, and filled a table with basic values.

Yet trying to access my app, I get:

Fatal error: Uncaught exception 'PDOException' with message
could not find driver' in /var/www/html/index.php:30 
Stack trace: #0 [internal function]: 
PDO->__construct('mysql:host=mysq...', 'root', 'root', Array) 
#1 [internal function]: Phalcon\Db\Adapter\Pdo->connect(Array)
#2 /var/www/html/index.php(30): Phalcon\Db\Adapter\Pdo-__construct(Array)
#3 [internal function]: {closure}()
#4 [internal function]: Phalcon\Di\Service->resolve(NULL, Object(Phalcon\Di\FactoryDefault))
#5 [internal function]: Phalcon\Di->get('db', NULL)
#6 [internal function]: Phalcon\Di->getShared('db')
#7 [internal function]: Phalcon\Mvc\Model\Manager->_getConnection(Object(Reviews), NULL)
#8 [internal function]: Phalcon\Mvc\Model\Manager->getReadConnection(Object(Reviews))
#9 [internal function]: Phalcon\Mvc\Model->getReadConnection()
#10 [internal function]: Phalcon\Mvc\Model\MetaData\Strategy\Introspection->getMetaData(Object(Reviews), Object(Phalcon\Di\FactoryDefault))
#11 [internal function]: Phalcon\Mvc\Model\MetaData->_initialize(Object(Rev in /var/www/html/index.php on line 30

Hence, I thought that the php container was lacking the php-mysql component I installed via:

apt-get install php5-mysql

I also added a mysql.ini at:

cat /usr/local/etc/php/conf.d/mysql.ini
; configuration for php MySQL module
; priority=20
extension=pdo_mysql.so

If I echo phpinfo();die it tells me that:

Additional .ini files parsed:
    /usr/local/etc/php/conf.d/mysql.ini,
    /usr/local/etc/php/conf.d/phalcon.ini

Yet still, the error persists.

Furthermore, when running:

php -i|grep PDO

I get:

PHP Warning:  PHP Startup: Unable to load dynamic library '/usr/local/lib/php/extensions/no-debug-non-zts-20131226/pdo_mysql.so' - /usr/local/lib/php/extensions/no-debug-non-zts-20131226/pdo_mysql.so: cannot open shared object file: No such file or directory in Unknown on line 0
PDO
PDO support => enabled
PDO drivers => sqlite
PDO Driver for SQLite 3.x => enabled

so it seems the mysql extension isn't even activated.

What am I doing wrong?

The official repository of PHP has a script called docker-php-ext-install https://github.com/docker-library/php/tree/master/5.6

You forgot to install the extension needed to run the PDO.

Try do create a docker image like this:

FROM php:5.6-apache

# PHP extensions

RUN docker-php-ext-install pdo pdo_mysql