fix: remove distribution-specific logic from Containerfile

The Containerfile should be generic and not contain conditionals tied to
specific distributions. Instead, users specify which config to use via
command-line arguments.

Changes:
- Removed ENABLE_POSTGRES_STORE conditional from entrypoint
- Updated docs to show proper usage: passing config as argument
  Example: llama stack run starter::run-with-postgres-store.yaml
- Maintains generic, reusable container entrypoint

This follows the principle of separation of concerns - the container
provides the runtime, while the user specifies the configuration.

containers/Containerfile

@@ -135,11 +135,6 @@ RUN cat <<'EOF' >/usr/local/bin/llama-stack-entrypoint.sh
 #!/bin/sh
 set -e
 
-# Check if ENABLE_POSTGRES_STORE is set to switch to postgres config
-if [ "${ENABLE_POSTGRES_STORE:-}" = "1" ] && [ "$DISTRO_NAME" = "starter" ]; then
-  exec llama stack run "starter::run-with-postgres-store.yaml" "$@"
-fi
-
 if [ -n "$RUN_CONFIG_PATH" ] && [ -f "$RUN_CONFIG_PATH" ]; then
   exec llama stack run "$RUN_CONFIG_PATH" "$@"
 fi