Add Authorization & Origin headers to traefik reverse proxy - docker-compose

Question

I have a very simple setup with with Traefik using Docker & Let's Encrypt to proxy incoming requests to an API service.
All is up and running well except that client's can't make requests to the API because of CORS.
How can I add Origin, Content-Type, and Authorization headers so I can ensure they reach my Node API running behind the proxy and ensure Access-Control-Allow-Origin are kept in the response headers?
My traefik.toml file:
debug = false
logLevel = "ERROR"
defaultEntryPoints = ["https","http"]
[entryPoints]
[entryPoints.http]
address = ":80"
[entryPoints.http.redirect]
entryPoint = "https"
[entryPoints.https]
address = ":443"
[entryPoints.https.tls]
[retry]
[docker]
endpoint = "unix:///var/run/docker.sock"
domain = "<hostname-here>"
watch = true
exposedByDefault = false
[acme]
email = "<your-email-here>"
storage = "acme.json"
entryPoint = "https"
onHostRule = true
[acme.httpChallenge]
entryPoint = "http"
My docker-compose.yml that runs my API:
version: "2"
services:
app:
build:
context: .
dockerfile: Dockerfile-App
environment:
- NODE_ENV=${NODE_ENV}
restart: always
networks:
- web
- default
expose:
- "5000"
labels:
- "traefik.docker.network=web"
- "traefik.enable=true"
- "traefik.basic.frontend.rule=Host:${HOSTNAME}"
- "traefik.frontend.auth.forward.trustForwardHeader=true"
- "traefik.frontend.passHostHeader=true"
- "traefik.basic.port=5000"
- "traefik.basic.protocol=http"
networks:
web:
external: true
The docs imply you can add customHeaders
traefik.frontend.headers.customResponseHeaders=EXPR Appends the headers to each response returned by the container, before forwarding the response to the client.
Format: HEADER:value||HEADER2:value2
But how am I supposed to know the value of dynamic values such as Authorization Tokens?
EDIT:
I tried adding a custom header like this:
- "traefik.frontend.headers.customResponseHeaders=Access-Control-Allow-Origin: *"
But that's not being appended to the response headers at all.

Answer 1

entrypoint > Router > Middleware > Service
I think you should create the middleware first then attach it to the specific router:
I got the headers added to the response by adding these labels to my service :
...
- "traefik.http.routers.frontend.entrypoints=http"
#creating middleware for headers
- "traefik.http.middlewares.frontend.headers.customresponseheaders.Access-Control-Allow-Methods=POST, GET, PUT, OPTIONS, DELETE"
- "traefik.http.middlewares.frontend.headers.customresponseheaders.Access-Control-Allow-Origin=*"
- "traefik.http.middlewares.frontend.headers.customresponseheaders.Access-Control-Allow-Headers=x-requested-with, Content-Type,Authorization"
#attach middleware to the frontend router
- "traefik.http.routers.frontend.middlewares=frontend"

Answer 2

I was conditionally resolving CORS in my nodejs application based on the Origin of the request in an effort to white-list host names, but the origin wasn't being set by Traefik properly. I removed the conditional and white-listed host names with Traefik instead.
After doing this, CORS works properly and my labels above are sound.