feat(builder): Implement build cache for manifests & layers

Implements a cache that keeps track of:

a) Manifests that have already been built (for up to 6 hours)
b) Layers that have already been seen (and uploaded to GCS)

This significantly speeds up response times for images that are full
or partial matches with previous images served by an instance.

tools/nixery/server/main.go

@@ -125,6 +125,7 @@ type registryHandler struct {
 	cfg    *config.Config
 	ctx    *context.Context
 	bucket *storage.BucketHandle
+	cache  *builder.BuildCache
 }
 
 func (h *registryHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
@@ -140,7 +141,7 @@ func (h *registryHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		imageTag := manifestMatches[2]
 		log.Printf("Requesting manifest for image %q at tag %q", imageName, imageTag)
 		image := builder.ImageFromName(imageName, imageTag)
-		buildResult, err := builder.BuildImage(h.ctx, h.cfg, &image, h.bucket)
+		buildResult, err := builder.BuildImage(h.ctx, h.cfg, h.cache, &image, h.bucket)
 
 		if err != nil {
 			writeError(w, 500, "UNKNOWN", "image build failure")
@@ -192,6 +193,7 @@ func main() {
 	cfg := config.FromEnv()
 	ctx := context.Background()
 	bucket := prepareBucket(&ctx, cfg)
+	cache := builder.NewCache()
 
 	log.Printf("Starting Kubernetes Nix controller on port %s\n", cfg.Port)
 
@@ -200,6 +202,7 @@ func main() {
 		cfg:    cfg,
 		ctx:    &ctx,
 		bucket: bucket,
+		cache:  &cache,
 	})
 
 	// All other roots are served by the static file server.