[CICD] add simple test step for docker build workflow, fix prefix bug (#821)

# What does this PR do?

**Main Thing**
- Add a simple test step before publishing docker image in workflow

**Side Fix**
- Docker push action fails recently due to extra prefix introduced. E.g.
see:
https://github.com/meta-llama/llama-stack/pull/802#issuecomment-2599507062

cc @terrytangyuan 

## Test Plan

1. Release a TestPyPi version on this code: 0.0.63.dev51206766


3581203331

```
# 1. build docker image
TEST_PYPI_VERSION=0.0.63.dev51206766 llama stack build --template fireworks

# 2. test the docker image
cd distributions/fireworks && docker compose up
```

4. Test the full build + test docker flow using TestPyPi from (1):
1284218494

<img width="1049" alt="image"
src="https://github.com/user-attachments/assets/c025893d-5ce2-48ff-aa90-de00e105ee09"
/>


## Sources

Please link relevant resources if necessary.


## Before submitting

- [ ] This PR fixes a typo or improves the docs (you can dismiss the
other checks if that's the case).
- [ ] Ran pre-commit to handle lint / formatting issues.
- [ ] Read the [contributor
guideline](https://github.com/meta-llama/llama-stack/blob/main/CONTRIBUTING.md),
      Pull Request section?
- [ ] Updated relevant documentation.
- [ ] Wrote necessary unit or integration tests.

distributions/together/compose.yaml

@@ -1,13 +1,11 @@
 services:
   llamastack:
     image: llamastack/distribution-together
-    network_mode: "host"
-    volumes:
-      - ~/.llama:/root/.llama
-      - ./run.yaml:/root/llamastack-run-together.yaml
     ports:
       - "8321:8321"
-    entrypoint: bash -c "python -m llama_stack.distribution.server.server --yaml_config /root/llamastack-run-together.yaml"
+    environment:
+      - TOGETHER_API_KEY=${TOGETHER_API_KEY}
+    entrypoint: bash -c "python -m llama_stack.distribution.server.server --template together"
     deploy:
       restart_policy:
         condition: on-failure