Only auto-detect platform variant if we are also auto-detecting the architecture

Otherwise, on ARM, users with ArchitectureChoice: "amd64" still end up with
a wantedVariant: "v7".

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

Author: mtrmac

internal/pkg/platform/platform_matcher.go

@@ -125,10 +125,17 @@ var compatibility = map[string][]string{
 // If some option (arch, os, variant) is not present, a value from current platform is detected.
 func WantedPlatforms(ctx *types.SystemContext) ([]imgspecv1.Platform, error) {
 	wantedArch := runtime.GOARCH
+	wantedVariant := ""
 	if ctx != nil && ctx.ArchitectureChoice != "" {
 		wantedArch = ctx.ArchitectureChoice
+	} else {
+		// Only auto-detect the variant if we are using the default architecture.
+		// If the user has specified the ArchitectureChoice, don't autodetect, even if
+		// ctx.ArchitectureChoice == runtime.GOARCH, because we have no idea whether the runtime.GOARCH
+		// value is relevant to the use case, and if we do autodetect a variant,
+		// ctx.VariantChoice can't be used to override it back to "".
+		wantedVariant = getCPUVariant(runtime.GOOS, runtime.GOARCH)
 	}
-	wantedVariant := getCPUVariant(runtime.GOOS, runtime.GOARCH)
 	if ctx != nil && ctx.VariantChoice != "" {
 		wantedVariant = ctx.VariantChoice
 	}