Fix "module not found" error in Docker
diff --git a/docker/docker-compose.yml b/docker/docker-compose.yml
index b3eda7f62b..66aad3be60 100644
--- a/docker/docker-compose.yml
+++ b/docker/docker-compose.yml
@@ -8,7 +8,7 @@ services:
     image: fastchat:latest
     ports:
       - "21001:21001"
-    entrypoint: ["python3", "-m", "fastchat.serve.controller", "--host", "0.0.0.0", "--port", "21001"]
+    entrypoint: ["python3.9", "-m", "fastchat.serve.controller", "--host", "0.0.0.0", "--port", "21001"]
   fastchat-model-worker:
     build:
       context: .
@@ -25,7 +25,7 @@ services:
             - driver: nvidia
               count: 1
               capabilities: [gpu]
-    entrypoint: ["python3", "-m", "fastchat.serve.model_worker", "--model-name", 'fastchat-t5-3b-v1.0', "--model-path", "lmsys/fastchat-t5-3b-v1.0", "--worker-address", "http://fastchat-model-worker:21002", "--controller-address", "http://fastchat-controller:21001", "--host", "0.0.0.0", "--port", "21002"]
+    entrypoint: ["python3.9", "-m", "fastchat.serve.model_worker", "--model-name", 'fastchat-t5-3b-v1.0', "--model-path", "lmsys/fastchat-t5-3b-v1.0", "--worker-address", "http://fastchat-model-worker:21002", "--controller-address", "http://fastchat-controller:21001", "--host", "0.0.0.0", "--port", "21002"]
   fastchat-api-server:
     build:
       context: .
@@ -35,6 +35,6 @@ services:
     image: fastchat:latest
     ports:
       - "8000:8000"
-    entrypoint: ["python3", "-m", "fastchat.serve.openai_api_server", "--controller-address", "http://fastchat-controller:21001", "--host", "0.0.0.0", "--port", "8000"]
+    entrypoint: ["python3.9", "-m", "fastchat.serve.openai_api_server", "--controller-address", "http://fastchat-controller:21001", "--host", "0.0.0.0", "--port", "8000"]
 volumes:
   huggingface:

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## Why are these changes needed?
When attempting to run via Docker Compose on a fresh Linux install with GPU enabled, I got the following error.
![image](https://github.com/lm-sys/FastChat/assets/3521582/85dc23d1-388f-4951-97c2-ccb5213d024d)
This was because by default, the container tries to use Python 3.8, though we want 3.9 to properly run. This enforces 3.9 for the containers in compose.
<!-- Please give a short summary of the change and the problem this solves. -->

## Related issue number (if applicable)
N/A
<!-- For example: "Closes #1234" -->

## Checks

- [x] I've run `format.sh` to lint the changes in this PR.
- [x] I've included any doc changes needed.
- [x] I've made sure the relevant tests are passing (if applicable).
