Finish building Postgres support (#133)

This includes:
 - Building a Docker image for Postgres as well as SQLite
 - Fuller instructions for usage of the package, including the Postgres builds.

A few related things changed here:
 - `.env` is not used anymore -- the defaults in the Dockerfiles are
   sufficient
 - The Rust version in the Dockerfiles is increased to match the MSRV,
   and with it the Alpine version bumped to one built with that Rust
   version.
 - Cargo dependencies on native-tls and openssl updated to include only
   the `vendored` feature, so as not to require a system openssl
   installation.
 - Two GitHub jobs are set up, to build the two different Docker images
 - The documentation incorrectly suggested using `DELETE .. CASCADE` to
   delete clients. This syntax does not exist, as the cascading delete
   is configured in the schema.

docs/src/usage/docker-compose.md

@@ -0,0 +1,43 @@
+# Docker Compose
+
+The
+[`docker-compose.yml`](https://raw.githubusercontent.com/GothenburgBitFactory/taskchampion-sync-server/refs/tags/v0.6.1/docker-compose.yml)
+file in this repository is sufficient to run taskchampion-sync-server,
+including setting up TLS certificates using Lets Encrypt, thanks to
+[Caddy](https://caddyserver.com/). This setup uses the SQLite backend, which is
+adequate for one or a few clients.
+
+You will need a server with ports 80 and 443 open to the Internet and with a
+fixed, publicly-resolvable hostname. These ports must be available both to your
+Taskwarrior clients and to the Lets Encrypt servers.
+
+On that server, download `docker-compose.yml` from the link above (it is pinned
+to the latest release) into the current directory. Then run
+
+```sh
+TASKCHAMPION_SYNC_SERVER_HOSTNAME=taskwarrior.example.com \
+TASKCHAMPION_SYNC_SERVER_CLIENT_ID=your-client-id \
+docker compose up
+```
+
+The `TASKCHAMPION_SYNC_SERVER_CLIENT_ID` limits the server to the given client
+ID; omit it to allow all client IDs. You may specify multiple client IDs
+separated by commas.
+
+It can take a few minutes to obtain the certificate; the caddy container will
+log a message "certificate obtained successfully" when this is complete, or
+error messages if the process fails. Once this process is complete, configure
+your `.taskrc`'s to point to the server:
+
+```none
+sync.server.url=https://taskwarrior.example.com
+sync.server.client_id=your-client-id
+sync.encryption_secret=your-encryption-secret
+```
+
+The docker-compose images store data in a docker volume named
+`taskchampion-sync-server_data`. This volume contains all of the task data, as
+well as the TLS certificate information. It will persist over restarts, in a
+typical Docker installation. The docker containers will start automatically
+when the Docker dameon starts. See the docker-compose documentation for more
+information.