Docker Compose: Make model endpoint URLs configurable via env vars

Parent: #758 (closed)

Summary

Switch ALL hardcoded Docker service names in docker-compose.yaml environment sections to ${VAR:-default} patterns. This covers both OPEA/AI services AND core services (ArangoDB, Backend, Doc-repo, Redis, Keycloak).

This issue covers the docker-compose.yaml environment sections only. Separate issues cover:

• Kong config: #766
• ChatQnA Python code: #759 (closed)
• Docling microservice: #764 (closed)

Services to Update

OPEA/AI Services

ChatQnA — add _URL env var passthrough:

chatqna-xeon-backend-server:
  environment:
    LLM_SERVER_URL: ${LLM_SERVER_URL:-}
    LLM_SERVER_HOST_IP: ${LLM_SERVER_HOST_IP:-vllm}
    LLM_SERVER_PORT: ${LLM_SERVER_PORT:-8000}
    # ... same for EMBEDDING, RETRIEVER, RERANK, TRANSLATION, GUARDRAIL

Dataprep — configurable model + core endpoints:

dataprep-arango-service:
  environment:
    VLLM_ENDPOINT: ${VLLM_ENDPOINT:-http://vllm:8000}
    TEI_EMBEDDING_ENDPOINT: ${TEI_EMBEDDING_ENDPOINT:-http://tei:80}
    GUARDRAIL_URL: ${GUARDRAIL_URL:-http://guardrail:9090/v1/guardrails}
    DOCLING_ENDPOINT: ${DOCLING_ENDPOINT:-}

Retriever:

retriever-arango-service:
  environment:
    VLLM_ENDPOINT: ${VLLM_ENDPOINT:-http://vllm:8000}
    TEI_EMBEDDING_ENDPOINT: ${TEI_EMBEDDING_ENDPOINT:-http://tei:80}

Microservice wrappers:

embedding:
  environment:
    TEI_EMBEDDING_ENDPOINT: ${TEI_EMBEDDING_ENDPOINT:-http://tei:80}

reranker:
  environment:
    TEI_RERANKING_ENDPOINT: ${TEI_RERANKING_ENDPOINT:-http://tei_reranker:80}

translation:
  environment:
    LLM_ENDPOINT: ${VLLM_TRANSLATION_ENDPOINT:-http://vllm-translation-guardrail:9031}

Core Services (see #765 for details)

ArangoDB URL (5 references):

- ARANGO_URL=${ARANGO_URL:-http://arango-vector-db:8529}

Backend URL (2 references in OPEA services):

BACKEND_SERVICE_URL: ${BACKEND_SERVICE_URL:-http://backend:3000}

Document Repository URL (2 references):

DOCUMENT_REPOSITORY_URL: ${DOCUMENT_REPOSITORY_URL:-http://document-repository:3001}
DOC_REPO_URL: ${DOC_REPO_URL:-http://document-repository:3001}

Keycloak Internal URL (keycloak-config):

- KEYCLOAK_URL=${KEYCLOAK_INTERNAL_URL:-http://keycloak:8080}

Redis (already uses env var):

- TRANSLATION_CACHE_HOST=${TRANSLATION_CACHE_HOST:-redis-cache}

Acceptance Criteria

• All model endpoint env vars use ${VAR:-default} pattern
• All core service URL env vars use ${VAR:-default} pattern
• DOCLING_ENDPOINT added to dataprep environment
• Defaults preserve current Docker DNS names (no behavior change)
• docker compose config validates without errors