fix impossibility to rebuild only child images

The Filter function rebuild a completely new graph were unmodified images are
completely discarded. I suppose it was done to have a (very) smaller graph to
improve (a little) performance.

Unfortunately, we need info on all direct parents images when rebuilding, at
least to properly amend the FROM line to be able to pull the right parent
image. Thus this refacto broke completely rebuild of every image with at least
one not-to-rebuild parent.

I fix this quickly by avoiding the Filter function altogether.

If the graph reduction were deemed useful it would be needed to discard only
nodes which are not modified *and* which have no modified descendants. But I
felt it was to complicated for me.

Fixes be5509e

pkg/dib/build.go

@@ -83,13 +83,13 @@ func (p *Builder) rebuildGraph(
 	buildArgs map[string]string,
 ) {
 	p.Graph.
-		Filter(
-			func(node *dag.Node) bool {
-				return node.Image.NeedsRebuild || node.Image.NeedsTests
-			}).
 		WalkParallel(
 			func(node *dag.Node) {
 				img := node.Image
+				if !(img.NeedsRebuild || img.NeedsTests) {
+					img.RebuildFailed = false
+					return
+				}
 				buildReport := report.BuildReport{Image: *img}
 
 				// Return if any parent build failed