[Autoscaler] Precisely match docker HOME (#12020)

* [Autoscaler] Precisely match docker HOME

The current grep will match any env variable keyed by HOME. This will
include some unwanted variables like PYTHONHOME, PROJECT_HOME, etc.
Depending on the order of the environment variable, the subsequent
docker setup command might fail.

* fstring

python/ray/autoscaler/_private/command_runner.py

@@ -669,8 +669,7 @@ class DockerCommandRunner(CommandRunnerInterface):
         if user_pos > -1:
             if self.home_dir is None:
                 self.home_dir = self.ssh_command_runner.run(
-                    "docker exec {} env | grep HOME | cut -d'=' -f2".format(
-                        self.container_name),
+                    f"docker exec {self.container_name} printenv HOME",
                     with_output=True).decode("utf-8").strip()
 
             if any_char: