Copy devcontainer from Symfony Docker scaffold - #3000

Summary

This copies the .devcontainer directory from the pinned dunglas/symfony-docker snapshot when --with-docker is used.

Scope choice for #2994

The issue mentions several possible migration gaps from the old template:

• A. Copy the upstream .devcontainer directory from symfony-docker.
• B. Add Docker Compose services for the generated PWA/admin apps.
• C. Add an explicit database service/configuration to the generated compose setup.
• D. Reintroduce or generate Helm chart assets.

This PR chooses A because the installer already imports Docker infrastructure from a pinned symfony-docker commit, and that same pinned snapshot contains .devcontainer. Copying it keeps ownership with the upstream Docker template and avoids inventing local PWA/admin/Postgres/Helm behavior before maintainers agree on those larger choices.

Test verification (RED → GREEN)

RED, before the fix, after adding the regression test:

...F...                                                             7 / 7 (100%)

1) ApiPlatform\Installer\Tests\Scaffold\SymfonyScaffoldTest::testSymfonyDockerCopyListIncludesDevcontainer
Failed asserting that an array contains '.devcontainer'.

FAILURES!
Tests: 7, Assertions: 15, Failures: 1.

GREEN, with the fix:

...............................................................  63 / 109 ( 57%)
..............................................                  109 / 109 (100%)

OK (109 tests, 219 assertions)

Fix-reverted regression check:

...F...                                                             7 / 7 (100%)

ApiPlatform\Installer\Tests\Scaffold\SymfonyScaffoldTest::testSymfonyDockerCopyListIncludesDevcontainer
Failed asserting that an array contains '.devcontainer'.

FAILURES!
Tests: 7, Assertions: 15, Failures: 1.

Static analysis:

php vendor/bin/phpstan analyse
[OK] No errors

All commands were run inside ephemeral Docker containers with docker run --rm -u "$(id -u):$(id -g)" -v "$PWD":/app -w /app composer:2 ....