Docker port mapping and redirects

These days, I mostly do local development of web sites with Docker containers. I spin up a little cluster with docker compose that includes an nginx container for the web server and maybe containers for other servers.

Typically, I just let the nginx server run on port 80 and map it to something else on my laptop:

nginx:
  image: nginx:1.23.3
  container_name: nginx
  ports:
    - 8385:80
  volumes:
    - ../build/website:/usr/share/nginx/html
    - ../build/cache:/var/cache/nginx
    - ../build/nginx/nginx.conf:/etc/nginx/nginx.conf:ro
  stdin_open: true
  tty: true
  networks:
    - external_net

When that container is running, localhost:8385 on my laptop is nginx on port 80.

For one particular container, I got very odd behavior: sometimes following a link would lose the port number. I’d click in a link to http://localhost:8385/path and the browser would complain that http://localhost/path/ wasn’t found. I assumed this was just browser caching or something until I saw the same thing in a link checking script.

Once I knew it wasn’t just some weirdness in the browser, it didn’t take long to track down. If you’re observant, there’s a clue in the previous paragraph. The attempt to read /path, the name of a directory, is redirected on the server side to /path/ but because the server thinks it’s running in port 80, that’s http://localhost/path/.

There’s nothing profound here, and the fix is simply to run nginx in the container on port 8385 as well. Problem solved. Well, mostly. The browser is still acting a bit funny sometimes, but I really think that’s just some caching artifact or something now.