Add Symfony Example

[Symfony](https://symfony.com/) is a PHP framework for web projects.

I'd like to add an example that builds a generic 'hello-world' Symfony app and connects to SQLite in a separate container.

One interesting aspect (and one about which I wrestle with in the 'this-isn't-something-for-prod-therefore-why-is-it-done-in-acontainer' way) is the idea of doing Composer work inside Docker.

IMO, it would be better to do all the app _build_ process in a separate pipeline, then _run_ the app in Docker containers. Technically the _build_ process could be managed in some Docker containers... but you should _always_ run your app in the exact same way as Production. And Production should not have Composer, nor should it be involved in your build process (especially since that saps up resources!).

The issue names no target files, tests, or entry points. Start by reviewing the repository's existing Docker examples and determine how a Symfony hello-world app, Composer build step, and separate SQLite container should fit those conventions; done means a working example that builds and runs as described.

Written by the indexing model from the issue text.