docs: put the Docker quick start back in the README

Slimming it left no way to try ihasmail without leaving GitHub. Restored in
short form -- the four commands, the 2FA app-password note, and links to the
install and configure guides for TLS and the full environment.

ROADMAP.md's 2FA entry points back at that section again, rather than at the
install docs it was redirected to when the section was gone.

README.md

@@ -64,6 +64,25 @@ on the first call.
 - Still on 0.15? The last release that runs on it is tagged [`stalwart-0.15-support`](https://github.com/LINUXexpert-org/ihasmail/releases/tag/stalwart-0.15-support).
 - Upgrading? [stalwart-migrator](https://github.com/LINUXexpert-org/stalwart-migrator) does it in place, checkpointing every phase and validating afterwards. The live instance moved 0.15.5 → 0.16.19 with eight seconds of downtime and nothing lost.
 
+## Quick start (Docker)
+
+```bash
+cp .env.example .env
+# edit: STALWART_URL=https://mail.example.com  and  APP_SECRET=$(openssl rand -base64 48)
+docker compose up --build -d
+# → http://localhost:8080  (put Caddy/nginx in front for TLS; see Caddyfile.example / nginx.example.conf)
+```
+
+Users sign in with their Stalwart mailbox credentials. **An account with
+two-factor authentication needs an app password**, created in Stalwart's own
+settings — Stalwart accepts a TOTP code only through an OAuth flow and offers no
+password grant, so no client holding a username and password can exchange them
+plus a code for a token.
+
+Full instructions, TLS, and every environment variable:
+[Installing](https://docs.ihasmail.org/install/) ·
+[Configuring](https://docs.ihasmail.org/configure/).
+
 ## Architecture
 
 ```