sync: periodically retry if on-demand fails inline, closes #281

sync: don't return error on sync signatures, just skip them, closes #375
sync: sync signatures on demand
sync on demand: in case of parallel requests pull image just once, closes #344

Signed-off-by: Petu Eusebiu <peusebiu@cisco.com>

pkg/common/common_test.go

@@ -0,0 +1,17 @@
+package common_test
+
+import (
+	"testing"
+
+	. "github.com/smartystreets/goconvey/convey"
+	"zotregistry.io/zot/pkg/common"
+)
+
+func TestCommon(t *testing.T) {
+	Convey("test Contains()", t, func() {
+		first := []string{"apple", "biscuit"}
+		So(common.Contains(first, "apple"), ShouldBeTrue)
+		So(common.Contains(first, "peach"), ShouldBeFalse)
+		So(common.Contains([]string{}, "apple"), ShouldBeFalse)
+	})
+}