Fix Docker build, ship nidusctl in the image, document user setup via compose

- Bump the builder image to golang:1.25-alpine to match go.mod's "go
  1.25.0" requirement — the previous 1.22 base failed to build at all.
- Stop COPYing the git-ignored config.yaml (which doesn't exist in a
  fresh checkout, breaking CI/release builds); ship config.example.yaml
  as the image's default instead, same as it's already overridden via
  a volume mount in docker-compose.yaml.
- Build and include the nidusctl binary in the runtime image so users
  can create their first user/calendars/address books straight from a
  running container via `docker compose exec`, without installing Go.
- README: document that Docker workflow, mention the pre-built
  multi-arch images published by the new release CI, and drop the
  "Client configuration" section — how to configure a given DAV client
  is out of scope for this project's docs.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

README.md

@@ -90,31 +90,33 @@ docker run -p 8080:8080 \
   davserver
 ```
 
----
+The image also ships `nidusctl`, so once the container is running you can
+create your first user (and their calendars/address books) with
+`docker compose exec` — no need to install Go locally:
 
-## Client configuration
+```bash
+docker compose exec davserver nidusctl -config /app/config.yaml user create alice \
+  --display-name "Alice Smith" --email alice@example.com
+# (prompts for a password; use --password to skip the prompt, e.g. in scripts)
 
-### Apple Calendar / Contacts (macOS / iOS)
+docker compose exec davserver nidusctl -config /app/config.yaml calendar create alice personal
+docker compose exec davserver nidusctl -config /app/config.yaml addressbook create alice contacts
+```
 
-1. Go to **Settings → Calendar → Accounts → Add Account → Other → Add CalDAV Account**
-2. Enter:
-   - **Server**: `http://yourserver:8080`
-   - **Username**: `alice`
-   - **Password**: your plaintext password
-3. The app will auto-discover calendars at `/cal/`.
+### Pre-built images
 
-Same flow for **CardDAV** with Contacts app.
+Pushing a version tag (e.g. `v1.2.3`) or publishing a release triggers
+[`.github/workflows/docker-release.yml`](.github/workflows/docker-release.yml),
+which builds and publishes a multi-arch (`linux/amd64` + `linux/arm64`)
+image to this repository's own container registry, tagged with both the
+version and `latest`. Point `docker-compose.yaml`'s `image:` at it instead
+of `build: .` to use it directly, e.g.:
 
-### Thunderbird
-
-1. Install the **TbSync** add-on + **CalDAV & CardDAV** provider
-2. Add a new account and point it at `http://yourserver:8080/.well-known/caldav`
-
-### GNOME Calendar / Evolution
-
-Use the GNOME Online Accounts panel:
-- **Server**: `http://yourserver:8080`
-- Check *CalDAV* / *CardDAV* as appropriate
+```yaml
+services:
+  davserver:
+    image: <registry>/<owner>/<repo>:latest
+```
 
 ---