build: use correct values for proxy variables (#10908)

clone variable before we capture a pointer

Signed-off-by: Nicolas De Loof <nicolas.deloof@gmail.com>

pkg/compose/build.go

@@ -354,6 +354,7 @@ func resolveAndMergeBuildArgs(
 	// so they're handled last
 	for k, v := range storeutil.GetProxyConfig(dockerCli) {
 		if _, ok := result[k]; !ok {
+			v := v
 			result[k] = &v
 		}
 	}