Traefik > "Bad gateway" (error 502) for some containers

The traefik port should be the http port of the container, not the published port on the host. It communicates over the docker network, so publishing the port is unnecessary and against the goals of only having a single port published with a reverse proxy to access all the containers.

In short, you need:

traefik.port=80

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Since this question has gotten lots of views, the other reason lots of people see a 502 from traefik is placing the containers on a different docker network from the traefik instance, or having a container on multiple networks and not telling traefik which network to use. This doesn't apply in your case since you have the following lines in your compose file that match up with the traefik service's network:

services:
  dokuwiki:
    networks:
      - traefik-network
    labels:
      - traefik.docker.network=traefik-network
networks:
  traefik-network:
    external : true

Even if you only assign a service to a single network, some actions like publishing a port will result in your service being attached to two different networks (the ingress network being the second). The network name in the label needs to be the external name, which in your case is the same, but for others that do not specify their network as external, it may have a project or stack name prefixed which you can see in the docker network ls output.

traefik.docker.network must also be the fully qualified network name. Either externally defined, or prefixed with the stack name.

You can alternatively define a default network with providers.docker.network=traefik-network which means you don't have to add the label to every container.