feat: add multi-stage Dockerfile and docker-compose.yml

- Multi-stage Dockerfile: golang:1.24-alpine builder → alpine:3.21 runtime
  - CGO_ENABLED=0 for static binary, stripped with -ldflags="-s -w"
  - Only copies ca-certificates and tzdata to runtime image
  - Config via volume mount at /etc/gosearch/config.toml
- docker-compose.yml: gosearch + Valkey 8
  - Valkey healthcheck ensures gosearch starts after cache is ready
  - Persistent Valkey volume
  - config.toml mounted read-only
- Update .gitignore with Go build artifacts

docker-compose.yml

@@ -0,0 +1,33 @@
+# Docker Compose for gosearch + Valkey
+#
+# Usage:
+#   cp config.example.toml config.toml   # edit as needed
+#   docker compose up -d
+
+services:
+  gosearch:
+    build: .
+    ports:
+      - "8080:8080"
+    volumes:
+      - ./config.toml:/etc/gosearch/config.toml:ro
+    depends_on:
+      valkey:
+        condition: service_healthy
+    restart: unless-stopped
+
+  valkey:
+    image: valkey/valkey:8
+    ports:
+      - "6379:6379"
+    volumes:
+      - valkey-data:/data
+    healthcheck:
+      test: ["CMD", "valkey-cli", "ping"]
+      interval: 5s
+      timeout: 3s
+      retries: 5
+    restart: unless-stopped
+
+volumes:
+  valkey-data: