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.

docker-compose.shared.yml

@@ -0,0 +1,50 @@
+# opencode-devbox docker-compose for shared machines
+#
+# For machines where multiple users share one OS account (e.g. 'garage').
+# Each user gets isolated config, data, and named volumes by setting
+# SIGNUM in their .env file.
+#
+# Setup per user:
+#   1. mkdir -p ~/<signum>/opencode-devbox && cd ~/<signum>/opencode-devbox
+#   2. cp docker-compose.shared.yml docker-compose.yml
+#   3. cp .env.shared.example .env
+#   4. Edit .env with your signum, provider, keys, etc.
+#   5. mkdir -p ~/<signum>/.config/opencode
+#   6. docker compose up -d
+#
+# Named volumes are automatically isolated per user because Docker Compose
+# prefixes them with the project directory name (e.g. opencode-devbox_devbox-data).
+# Since each user runs from ~/<signum>/opencode-devbox/, volumes don't collide.
+
+services:
+  devbox:
+    image: joakimp/opencode-devbox:latest
+    container_name: devbox-${SIGNUM:?Set SIGNUM in .env}
+    stdin_open: true
+    tty: true
+    env_file:
+      - .env
+    environment:
+      - TERM=xterm-256color
+    volumes:
+      # Host workspace — user's project directory
+      - ${WORKSPACE_PATH:-~/src}:/workspace
+
+      # SSH keys — user-specific if available, else shared
+      - ${SSH_KEY_PATH:-~/.ssh}:/home/developer/.ssh:ro
+
+      # Opencode config — per-user (persists settings across restarts)
+      - ${HOME}/${SIGNUM}/.config/opencode:/home/developer/.config/opencode
+
+      # Persist opencode data (auth, memory, session history)
+      - devbox-data:/home/developer/.local/share/opencode
+
+      # Persist uv data (Python installs)
+      - devbox-uv:/home/developer/.local/share/uv
+
+      # Optional: AWS credentials (per-user if available)
+      # - ${HOME}/${SIGNUM}/.aws:/home/developer/.aws
+
+volumes:
+  devbox-data:
+  devbox-uv: