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

.gitignore

@@ -1 +1,11 @@
+node_modules/
+.agent/
+*.exe
+*.exe~
+*.dll
+*.so
+*.dylib
+*.test
+*.out
+gosearch
 config.toml