implement first pass at cached image data source (#3)

implements envbuilder_cached_image data source

testutil/registrytest/registrytest.go

@@ -0,0 +1,24 @@
+package registrytest
+
+import (
+	"fmt"
+	"net/http/httptest"
+	"net/url"
+	"testing"
+
+	"github.com/google/go-containerregistry/pkg/registry"
+	"github.com/stretchr/testify/require"
+)
+
+// New starts a new Docker registry listening on localhost.
+// It will automatically shut down when the test finishes.
+// It will store data in dir.
+func New(t testing.TB, dir string) string {
+	t.Helper()
+	regHandler := registry.New(registry.WithBlobHandler(registry.NewDiskBlobHandler(dir)))
+	regSrv := httptest.NewServer(regHandler)
+	t.Cleanup(func() { regSrv.Close() })
+	regSrvURL, err := url.Parse(regSrv.URL)
+	require.NoError(t, err)
+	return fmt.Sprintf("localhost:%s", regSrvURL.Port())
+}