Add shared-machine multi-user setup with per-user isolation via SIGNUM

For machines where multiple users share one OS account. Each user gets
isolated containers, config, and named volumes by running docker compose
from their own directory with a unique SIGNUM in .env.

README.md

@@ -271,6 +271,39 @@ volumes:
   - devbox-vscode:/home/developer/.vscode-server
 ```
 
+### Shared machine setup (multiple users, single OS account)
+
+For machines where multiple users share one OS account (e.g. a common `garage` user), a separate compose file isolates each user's config and data using a `SIGNUM` variable.
+
+Each user creates their own directory and setup:
+
+```bash
+# Replace <signum> with your username/identifier
+mkdir -p ~/<signum>/opencode-devbox
+cd ~/<signum>/opencode-devbox
+
+# Copy the shared-machine compose and env files
+cp /path/to/opencode-devbox/docker-compose.shared.yml docker-compose.yml
+cp /path/to/opencode-devbox/.env.shared.example .env
+
+# Create per-user config directory
+mkdir -p ~/<signum>/.config/opencode
+
+# Edit .env with your signum, provider, keys, etc.
+vim .env
+
+# Start
+docker compose up -d
+docker compose exec -u developer devbox-<signum> opencode
+```
+
+Each user's container, config, and named volumes are fully isolated:
+- Container name: `devbox-<signum>` (no collisions)
+- Named volumes: prefixed with the project directory name (automatic per-user isolation)
+- Opencode config: `~/<signum>/.config/opencode/` (per-user settings, OMOS config, etc.)
+
+See `docker-compose.shared.yml` and `.env.shared.example` for the full configuration.
+
 ### Rebuilding the Image
 
 `docker compose run` and `docker compose up` use the existing image — they **do not rebuild** when you change the Dockerfile or build args (e.g. updating `OPENCODE_VERSION`). Rebuild explicitly: