Kong config: Make upstream host references configurable

Summary

Make the Kong API gateway configuration (kong_config.json) use configurable upstream host references instead of hardcoded Docker service names. This is needed for deployments where Kong is on a different node than the backend, document-repository, or keycloak.

Current State — Hardcoded Hosts

api-gateway-solution/new-config/kong_config.json:

{
  "services": [
    { "name": "express-api", "host": "backend", "port": 3000 },
    { "name": "document-repository", "host": "document-repository", "port": 3001 },
    { "name": "keycloak", "host": "keycloak", "port": 8080 }
  ],
  "targets": [
    { "target": "backend:3000", "upstream": {"id": "..."} }
  ]
}

These 4 references are hardcoded to Docker DNS names. If backend or doc-repo are on a different node than Kong, these won't resolve.

Options

Option A: Template kong_config.json with envsubst

Convert kong_config.json to a template (kong_config.json.template) with shell variable placeholders:

{
  "services": [
    { "name": "express-api", "host": "${KONG_BACKEND_HOST:-backend}", "port": ${KONG_BACKEND_PORT:-3000} },
    { "name": "document-repository", "host": "${KONG_DOCREPO_HOST:-document-repository}", "port": ${KONG_DOCREPO_PORT:-3001} },
    { "name": "keycloak", "host": "${KONG_KEYCLOAK_HOST:-keycloak}", "port": ${KONG_KEYCLOAK_PORT:-8080} }
  ],
  "targets": [
    { "target": "${KONG_BACKEND_HOST:-backend}:${KONG_BACKEND_PORT:-3000}" }
  ]
}

Then in docker-compose.yaml, the kong-config service runs envsubst before applying:

kong-config:
  command: ["sh", "-c", "envsubst < /opt/kong-config/kong_config.json.template > /tmp/kong_config.json && kong config db_import /tmp/kong_config.json"]

Option B: Kong declarative config with environment variable support

Kong's declarative config doesn't natively support env vars. Option A (envsubst) is the standard workaround.

Option C: Use Kong Admin API to register services dynamically

Replace the static JSON config with a startup script that calls the Kong Admin API to register each service with configurable hosts. More flexible but more complex.

Recommendation: Option A — simplest, standard pattern, no Kong knowledge required.

Implementation

Rename kong_config.json → kong_config.json.template
Replace hardcoded hosts with ${VAR:-default} placeholders
Update docker-compose.yaml kong-config service to run envsubst before applying
Pass KONG_BACKEND_HOST, KONG_DOCREPO_HOST, KONG_KEYCLOAK_HOST as environment vars

Files

File	Change
`api-gateway-solution/new-config/kong_config.json`	Rename to `.template`, add env var placeholders
`docker-compose.yaml`	Update kong-config command to run envsubst

Acceptance Criteria

Kong config uses env-var-with-default for all upstream hosts
Default behavior unchanged (no env vars = current Docker DNS names)
docker compose config validates without errors
Kong correctly routes to backend when KONG_BACKEND_HOST is overridden