Fix the target name issue for multi-stage build

This PR is trying to fix issue #36956.

The stage name is case-insensitive by design, so we should use
`strings.EqualFold()` as the comparison method to eliminate the
case sensitive noise.

Also we need to return a pre-defined error code order to avoid below
message like:
"FIXME: Got an API for which error does not match any expected type!!!:
failed to reach build target dev in Dockerfile"

Signed-off-by: Dennis Chen <dennis.chen@arm.com>

builder/dockerfile/builder.go

@@ -226,7 +226,7 @@ func (b *Builder) build(source builder.Source, dockerfile *parser.Result) (*buil
 		targetIx, found := instructions.HasStage(stages, b.options.Target)
 		if !found {
 			buildsFailed.WithValues(metricsBuildTargetNotReachableError).Inc()
-			return nil, errors.Errorf("failed to reach build target %s in Dockerfile", b.options.Target)
+			return nil, errdefs.InvalidParameter(errors.Errorf("failed to reach build target %s in Dockerfile", b.options.Target))
 		}
 		stages = stages[:targetIx+1]
 	}

builder/dockerfile/instructions/commands.go

@@ -390,7 +390,8 @@ func CurrentStage(s []Stage) (*Stage, error) {
 // HasStage looks for the presence of a given stage name
 func HasStage(s []Stage, name string) (int, bool) {
 	for i, stage := range s {
-		if stage.Name == name {
+		// Stage name is case-insensitive by design
+		if strings.EqualFold(stage.Name, name) {
 			return i, true
 		}
 	}