docker: make compose work with cgroups (v2), conversion scripts/docs

docs/DOCKER_INSTALLATION_EN.md

@@ -5,7 +5,13 @@
 - The Docker image is only suitable for amd64. If you need to run it on a different architecture, try modifying the Dockerfile (specifically the part responsible for installing dovecot).
 
 # Docker installation
-This section provides instructions for installing Chatmail using docker-compose.
+This section provides instructions for installing Chatmail using Docker Compose.
+
+**Note:** Docker Compose v2 is required (`docker compose`, not `docker-compose`) for its support of the `cgroup: host` option in `docker-compose.yaml` is only supported by Compose v2.
+[see documentation](https://docs.docker.com/engine/install/debian/#install-using-the-repository)
+```shell
+apt install docker-ce docker-compose-plugin docker.io- docker-compose-
+```
 
 ## Preliminary setup
 We use `chat.example.org` as the Chatmail domain in the following steps.
@@ -75,6 +81,9 @@ docker compose up -d # start service
 docker compose logs -f chatmail # view container logs, press CTRL+C to exit
 ```
 
+### venv creation
+The first container start takes longer because it creates the cmdeploy Python virtualenv at `/opt/chatmail/venv` (persisted on the host via volume mount). Subsequent starts reuse the existing venv. Set `RECREATE_VENV=true` in `.env` to force a rebuild if needed.
+
 6. After installation is complete, you can open `https://<your_domain_name>` in your browser.
 
 ## Using custom files
@@ -114,6 +123,50 @@ docker compose down
 docker compose up -d
 ```
 
+## Migrating from a bare-metal install
+
+If you have an existing bare-metal Chatmail installation and want to switch to Docker:
+
+1. Stop all existing services:
+
+```shell
+systemctl stop postfix dovecot doveauth nginx opendkim unbound acmetool-redirector \
+  filtermail filtermail-incoming chatmail-turn iroh-relay chatmail-metadata \
+  lastlogin mtail
+systemctl disable postfix dovecot doveauth nginx opendkim unbound acmetool-redirector \
+  filtermail filtermail-incoming chatmail-turn iroh-relay chatmail-metadata \
+  lastlogin mtail
+```
+
+2. Convert your existing `chatmail.ini` to the Docker `.env` format:
+
+```shell
+python3 docker/cm_ini_to_env.py /usr/local/lib/chatmaild/chatmail.ini .env
+```
+
+3. Copy persistent data into the `./data/` subdirectories:
+
+```shell
+mkdir -p data/chatmail-dkimkeys data/chatmail-acme data/chatmail
+
+# DKIM keys
+cp -a /etc/dkimkeys/* data/chatmail-dkimkeys/
+
+# ACME certificates and account
+rsync -a /var/lib/acme/ data/chatmail-acme/
+
+# Mail data
+rsync -a /home/ data/chatmail/
+```
+
+Alternatively, you can mount `/home/vmail` directly by changing the volume in `docker-compose.yaml`:
+
+```yaml
+- /home/vmail:/home/vmail
+```
+
+The three `./data/` subdirectories cover all persistent state. Everything else is regenerated by the `configure` and `activate` stages on container start.
+
 ## Forcing a full reinstall
 
 The Docker image bakes the install stage (binary downloads, package setup, chatmaild venv) into the image at build time. On container start, only the `configure` and `activate` stages run by default.