stage_executor: reset platform in systemcontext for stages

On processing of every stage platform spec in systemcontext must be
correctly reset.

[NO NEW TESTS NEEDED]

Closes: containers#5968

Signed-off-by: flouthoc <[email protected]>

Author: flouthoc

imagebuildah/stage_executor.go

@@ -953,19 +953,18 @@ func (s *StageExecutor) prepare(ctx context.Context, from string, initializeIBCo
 	}
 
 	builderSystemContext := s.executor.systemContext
-	// get platform string from stage
-	if stage.Builder.Platform != "" {
-		os, arch, variant, err := parse.Platform(stage.Builder.Platform)
-		if err != nil {
-			return nil, fmt.Errorf("unable to parse platform %q: %w", stage.Builder.Platform, err)
-		}
-		if arch != "" || variant != "" {
-			builderSystemContext.ArchitectureChoice = arch
-			builderSystemContext.VariantChoice = variant
-		}
-		if os != "" {
-			builderSystemContext.OSChoice = os
-		}
+	// stage.Builder.Platform should be empty if user did not set `FROM --platform=<>`
+	// explicitly for the base image of this image which should set platform to the default.
+	os, arch, variant, err := parse.Platform(stage.Builder.Platform)
+	if err != nil {
+		return nil, fmt.Errorf("unable to parse platform %q: %w", stage.Builder.Platform, err)
+	}
+	if arch != "" || variant != "" {
+		builderSystemContext.ArchitectureChoice = arch
+		builderSystemContext.VariantChoice = variant
+	}
+	if os != "" {
+		builderSystemContext.OSChoice = os
 	}
 
 	builderOptions := buildah.BuilderOptions{