fix: #2495 FileNotFound Err in container image (#2498)

# What does this PR do?

Closes #2495 

Changes:
- Delay the `COPY run.yaml` into docker image step until after external
provider handling
- Split the check for `external_providers_dir` into “non-empty” and
“directory exists"


## Test Plan

0. Create and Activate venv

1. Create a `simple_build.yaml`
    ```yaml
    version: '2'
    distribution_spec:
      providers:
        inference:
          - remote::openai
    image_type: container
    image_name: openai-stack
    ```

2. Run llama stack build:
```bash
llama stack build --config simple_build.yaml
```

3. Run the docker container:
```bash
docker run \
  -p 8321:8321 \
  -e OPENAI_API_KEY=$OPENAI_API_KEY \
  openai_stack:0.2.12
```

This should show server is running.
```
INFO     2025-06-23 19:07:57,832 llama_stack.distribution.distribution:151 core: Loading external providers from /.llama/providers.d
INFO     2025-06-23 19:07:59,324 __main__:572 server: Listening on ['::', '0.0.0.0']:8321
INFO:     Started server process [1]
INFO:     Waiting for application startup.
INFO     2025-06-23 19:07:59,336 __main__:156 server: Starting up
INFO:     Application startup complete.                                                                             
INFO:     Uvicorn running on http://['::', '0.0.0.0']:8321 (Press CTRL+C to quit)
```

Notice the first line:
```
Loading external providers from /.llama/providers.d
```
This is expected behaviour.

Co-authored-by: Rohan Awhad <rawhad@redhat.com>

llama_stack/distribution/build_container.sh

@@ -163,9 +163,6 @@ EOF
 if [ -n "$run_config" ]; then
   # Copy the run config to the build context since it's an absolute path
   cp "$run_config" "$BUILD_CONTEXT_DIR/run.yaml"
-  add_to_container << EOF
-COPY run.yaml $RUN_CONFIG_PATH
-EOF
 
   # Parse the run.yaml configuration to identify external provider directories
   # If external providers are specified, copy their directory to the container
@@ -173,12 +170,15 @@ EOF
   python_cmd=$(get_python_cmd)
   external_providers_dir=$($python_cmd -c "import yaml; config = yaml.safe_load(open('$run_config')); print(config.get('external_providers_dir') or '')")
   external_providers_dir=$(eval echo "$external_providers_dir")
-  if [ -n "$external_providers_dir" ] && [ -d "$external_providers_dir" ]; then
+  if [ -n "$external_providers_dir" ]; then
+    if [ -d "$external_providers_dir" ]; then
     echo "Copying external providers directory: $external_providers_dir"
     cp -r "$external_providers_dir" "$BUILD_CONTEXT_DIR/providers.d"
     add_to_container << EOF
 COPY providers.d /.llama/providers.d
 EOF
+    fi
+
     # Edit the run.yaml file to change the external_providers_dir to /.llama/providers.d
     if [ "$(uname)" = "Darwin" ]; then
       sed -i.bak -e 's|external_providers_dir:.*|external_providers_dir: /.llama/providers.d|' "$BUILD_CONTEXT_DIR/run.yaml"
@@ -187,6 +187,11 @@ EOF
       sed -i 's|external_providers_dir:.*|external_providers_dir: /.llama/providers.d|' "$BUILD_CONTEXT_DIR/run.yaml"
     fi
   fi
+
+  # Copy run config into docker image
+  add_to_container << EOF
+COPY run.yaml $RUN_CONFIG_PATH
+EOF
 fi
 
 stack_mount="/app/llama-stack-source"