Review of platform detection (#56)

* Leave default image platform empty for static provider (see FAQ doc)
* Handle platform variant
* Add database migration process
* Switch to Open Container Specification labels as label-schema.org ones are deprecated
* Remove unneeded `diun.os` and `diun.arch` docker labels

Co-authored-by: CrazyMax <crazy-max@users.noreply.github.com>

pkg/registry/registry_test.go

@@ -0,0 +1,28 @@
+package registry_test
+
+import (
+	"os"
+	"testing"
+
+	"github.com/crazy-max/diun/pkg/registry"
+	"github.com/stretchr/testify/assert"
+)
+
+var (
+	rc *registry.Client
+)
+
+func TestMain(m *testing.M) {
+	var err error
+
+	rc, err = registry.New(registry.Options{})
+	if err != nil {
+		panic(err.Error())
+	}
+
+	os.Exit(m.Run())
+}
+
+func TestNew(t *testing.T) {
+	assert.NotNil(t, rc)
+}