TL;DR: composer install fails when the post-install scripts can't see a MySQL server
I'm building a docker container for a Symfony application, and during the build I do something like this
RUN export SYMFONY_ENV=prod && \ composer install --prefer-dist --optimize-autoloader
Towards the end of the install, it fails with this
Generating optimized autoload files [Doctrine\DBAL\Exception\DriverException] An exception occured in driver: SQLSTATE[HY000]  Can't connect to MySQL server on '127.0.0.1' (111) [Doctrine\DBAL\Driver\PDOException] SQLSTATE[HY000]  Can't connect to MySQL server on '127.0.0.1' (111) [PDOException] SQLSTATE[HY000]  Can't connect to MySQL server on '127.0.0.1' (111) Script Sensio\Bundle\DistributionBundle\Composer\ScriptHandler::clearCache handling the post-install-cmd event terminated with an exception [RuntimeException] An error occurred when executing the "'cache:clear --no-warmup'" command.
Now, I could use
--no-scripts but then presumably I'd need to run
composer install again after starting a container, and I'd like my container to be as ready as possible.
As I've not found many references to similar problems, there might be something in my application at fault, and I suspect I'll be answering my own question. My hope was that in describing it, I would figure it out. Alas, I haven't. Clues are welcome :)