Odd issue with only Nitter giving connection refused for IPv6 behind Traefik v1 with docker.
Many tens of services all work fine, on both IPv4 and IPv6, however Nitter does not.

This is likely not the fault of Traefik, I'd suspect it to not be a problem in other configurations, where all traffic from the reverse proxy is forwarded to an IPv4 address.

`nitter.conf`:
```
[Server]
address = "0.0.0.0" # Changing this to anything really doesn't seem to affect anything. Can leave empty as well.
port = 8080
https = true  # disable to enable cookies when not using https
staticDir = "./public"
hostname = "nitter.net"
...
```

`docker-compose.yml`:
```
version: '3'

networks:
  web_proxy:
    external: true

services:
  traefik:
    image: 'traefik:1.7-alpine'
    container_name: traefik
    volumes:
      - '/var/run/docker.sock:/var/run/docker.sock'
      - './traefik.toml:/traefik.toml:ro'
      - './acme.json:/acme.json:rw'
    ports:
      - '80:80'
      - '443:443'
    network_mode: web_proxy
    
  nitter:
    image: 'zedeus/nitter'
    volumes:
      - ./nitter.conf:/src/nitter.conf:ro
    depends_on:
      - traefik
    labels:
      - traefik.frontend.rule=Host:nitter.net
      - traefik.port=8080
    networks:
      - web_proxy
 
  ipv6workshere:
    image: httpd:alpine
    depends_on:
      - traefik
    labels:
      - traefik.frontend.rule=Host:ipv6.nitter.net
      - traefik.port=80
    networks:
      - web_proxy
```
To show, that the config is otherwise fine, there's a httpd instance added at the end.

For full replication, you also need an (empty) acme.json file, `chmod 600`, and a `traefik.toml`:
```
defaultEntryPoints = ["http", "https"]

[entryPoints]
  [entryPoints.http]
    address = ":80"
      [entryPoints.http.redirect]
        entryPoint = "https"
  [entryPoints.https]
    address = ":443"

      [entryPoints.https.redirect]
      regex = "^https://www.(.*)"
      replacement = "https://$1"
      permanent = true
      [entryPoints.https.tls]
      compress = true

[acme]
email = "user@domain.tld"
storage = "acme.json"
entryPoint = "https"
onHostRule = true
  [acme.httpChallenge]
  entryPoint = "http"

[docker]
watch = true
network = "web_proxy"
```

As a debugging measure, adding:
```
    ports:
      - 5555:8080
```
Makes the container accessible over ipv4, but not over ipv6. When doing the same with httpd, it's available on both ipv4 and ipv6.
```
curl -6 http://nitter.net:5555
curl: (7) Failed to connect to nitter.net port 5555: Connection refused
```