imagetools: refactor combining repository logic

This patch modifies the existing combining code in imagetools create to
provide better support for multiple repositories down the road.
Specifically, the code should no longer rely on a single repository
being used for all sources and tags, and should resolve descriptors in
their relevant repositories.

Signed-off-by: Justin Chadwell <me@jedevc.com>

build/build.go

@@ -925,7 +925,21 @@ func BuildWithResultHandler(ctx context.Context, drivers []DriverInfo, opt map[s
 
 							itpull := imagetools.New(imageopt)
 
-							dt, desc, err := itpull.Combine(ctx, names[0], descs)
+							ref, err := reference.ParseNormalizedNamed(names[0])
+							if err != nil {
+								return err
+							}
+							ref = reference.TagNameOnly(ref)
+
+							srcs := make([]*imagetools.Source, len(descs))
+							for i, desc := range descs {
+								srcs[i] = &imagetools.Source{
+									Desc: desc,
+									Ref:  ref,
+								}
+							}
+
+							dt, desc, err := itpull.Combine(ctx, srcs)
 							if err != nil {
 								return err
 							}