chore: docker compose v2

The `docker-compose` command has been replaced by `docker compose`.
The "version" tag has also been deprecated in docker-compose.yml.

README.md

@@ -76,10 +76,13 @@ sudo ./spectre-meltdown-checker.sh
 #### With docker-compose
 
 ```shell
-docker-compose build
-docker-compose run --rm spectre-meltdown-checker
+docker compose build
+docker compose run --rm spectre-meltdown-checker
 ```
 
+Note that on older versions of docker, `docker-compose` is a separate command, so you might
+need to replace the two `docker compose` occurences above by `docker-compose`.
+
 #### Without docker-compose
 
 ```shell