docs: dev container and local database stack guide

[ZhuchkaTriplesix]

New guide docs/dev-containers.md, linked from docs/README.md and from Local dev databases in docs/getting-started.md.

What it covers

• what the dev container and the docker/ stack contain (files, post-create / post-start commands, ports, host requirements);
• opening it in VS Code, with the devcontainer CLI and in Codespaces;
• the database stack: start, stop, reset, .env overrides, a table of ports, credentials and seeded data;
• connecting Querya to each database from the host and from inside the container: localhost is the container itself, so the stack's network has to be joined (docker network connect querya-dev_default "$(hostname)") or the namespace shared; MySQL 8.4 needs Use SSL/TLS; MongoDB's auth source is admin;
• running the app (Xvfb, or X11 forwarding as an optional recipe) and the tests: sections, e2e, goldens, sandbox tests, the live MySQL test with a CI-like server, the extension tester and its exit codes;
• known gaps (the image is on Flutter 3.41.6, CI pins 3.47.1, the registry has no 3.47.x image; how to align) and a troubleshooting table.

Checked

Against the repository (devcontainer.json, Dockerfile, docker-compose.yml, ci.yml, test_sections.sh, the live test) and against the running stack: ports, credentials, seeded collections / tables, MySQL's caching_sha2_password user on 8.4, the compose network name querya-dev_default, and the available image tags.

Not checked

The guide's commands were not run inside a freshly built dev container: the X11 forwarding recipe, docker network connect from inside it, --network container:$(hostname) and the Flutter checkout in Known gaps follow standard Docker and git behaviour but are worth a quick run by whoever uses the container first.