refactor: use official golangci-lint image for lint stage

Restructure Dockerfile to match upaas/dnswatcher pattern:
- Separate lint stage using golangci/golangci-lint:v2.1.6 image
- Builder stage for tests and compilation (no lint dependency)
- Add fmt-check Makefile target
- Decouple test from lint in Makefile (lint runs in its own stage)
- Run gofmt on all files
- docker build verified passing locally

internal/secret/validation_test.go

@@ -154,4 +154,3 @@ func TestValidateGPGKeyID(t *testing.T) {
 		})
 	}
 }
-