build: Use normalized platform spec for ARG expansion

The ARG variables for multi-platform builds should use a normalized
platform spec. This ensures consistent formatting across different
platform string representations, particularly for edge cases like
architecture normalization.

--
https://github.com/containers/buildah/blob/c97d402ccd8dbc5442c3b9fd952fece3f22fbce4/internal/util/util.go#L44
https://github.com/containers/common/blob/a5ccdae846b629b5ceaefa6ffd5c6511409c3487/libimage/platform/platform.go#L20

Signed-off-by: Michal Šoltis <[email protected]>

Author: slimreaper35

pkg/commands/build.go

@@ -556,7 +556,7 @@ func (c *Build) parseContainerfile() (*dockerfile.Dockerfile, error) {
 func (c *Build) createBuildArgExpander() (dockerfile.SingleWordExpander, error) {
 	// Define built-in ARG variables
 	// See https://docs.docker.com/build/building/variables/#multi-platform-build-arguments
-	platform := platforms.DefaultSpec()
+	platform := platforms.Normalize(platforms.DefaultSpec())
 	args := map[string]string{
 		// We current don't explicitly expose the --platform flag, so the TARGET* values always
 		// match the BUILD* values. If we add --platform handling, we would want to respect it here.