Map Commands instead of using them as a slice

The most obvious use case is when one wants to make sure as fast
as possible that a command is a valid Dockerfile command.

Signed-off-by: kargakis <kargakis@users.noreply.github.com>
Upstream-commit: 6ecf23861ec7036aa4210fe6d3b7dbfaaedbd6d0
Component: engine

components/engine/integration-cli/docker_cli_build_test.go

@@ -4836,7 +4836,7 @@ func TestBuildMissingArgs(t *testing.T) {
 
 	defer deleteAllContainers()
 
-	for _, cmd := range command.Commands {
+	for cmd := range command.Commands {
 		cmd = strings.ToUpper(cmd)
 		if _, ok := skipCmds[cmd]; ok {
 			continue