progress: add prefix to vertex progress group

As buildkit now uses progress groups for the COPY --link instruction
we need to ensure that we additionally prefix the progress group name,
or the target name will be left off in bake commands with more than one
target.

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

util/progress/multiwriter.go

@@ -24,6 +24,9 @@ func (p *prefixed) Write(v *client.SolveStatus) {
 	if p.force {
 		for _, v := range v.Vertexes {
 			v.Name = addPrefix(p.pfx, v.Name)
+			if v.ProgressGroup != nil {
+				v.ProgressGroup.Name = addPrefix(p.pfx, v.ProgressGroup.Name)
+			}
 		}
 	}
 	p.Writer.Write(v)