refactor: add comments explaining Docker incompatibility reason

Signed-off-by: Ogulcan Aydogan <ogulcanaydogan@hotmail.com>

Author: ogulcanaydogan

cmd/nerdctl/container/multi_platform_linux_test.go

@@ -88,6 +88,7 @@ func TestMultiPlatformBuildPush(t *testing.T) {
 	testCase := nerdtest.Setup()
 
 	testCase.Require = require.All(
+		// non-buildx `docker build` lacks multi-platform support; `docker push` lacks --platform
 		require.Not(nerdtest.Docker),
 		nerdtest.Build,
 		requireMultiPlatformExec,
@@ -134,6 +135,7 @@ func TestMultiPlatformBuildPushNoRun(t *testing.T) {
 	testCase := nerdtest.Setup()
 
 	testCase.Require = require.All(
+		// non-buildx `docker build` lacks multi-platform support; `docker push` lacks --platform
 		require.Not(nerdtest.Docker),
 		nerdtest.Build,
 		requireMultiPlatformExec,