Add continuing to use traefik doc

docs/upgrading/continuing-to-use-traefik.md

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+---
+title: "Kamal 2: Continuing to use Traefik"
+---
+
+# Kamal 2: Continuing to use Traefik
+
+Kamal 2 requires kamal-proxy, but it's possible to continue to use Traefik if required.
+
+You can run it as a Kamal accessory, and route requests through it and then on to kamal-proxy.
+
+## Set the kamal-proxy boot config
+
+We'll need to change kamal-proxy's default boot config so that:
+
+1. It doesn't publish ports on the host
+1. It adds the labels Traefik needs to route requests to it
+
+Add a [pre-deploy hook](../../hooks/pre-deploy) for Traefik to pick up:
+
+```shell
+#!/bin/sh
+kamal proxy boot_config set \
+  --publish false \
+  --docker_options label=traefik.http.services.kamal_proxy.loadbalancer.server.scheme=http \
+                   label=traefik.http.routers.kamal_proxy.rule=PathPrefix\(\`/\`\)
+```
+
+You can add the `kamal proxy boot_config set` command to a [pre-deploy hook](../../hooks/pre-deploy). This will ensure that it is set correctly when deploying to a host for the first time.
+
+## Create the accessory
+
+Add Traefik as an accessory to `config/deploy.yml`, binding to the host port.
+
+```yaml
+accessories:
+  traefik:
+    service: traefik
+    image: traefik:v2.10
+    port: 80
+    cmd: "--providers.docker"
+    options:
+      volume:
+        - "/var/run/docker.sock:/var/run/docker.sock"
+    roles:
+      - web
+```
+
+## Running with Traefik
+
+When you call `kamal setup`, it will boot the Traefik accessory, call the pre-deploy hook to update kamal-proxy's boot config and then boot kamal-proxy and the app.
+
+Requests will flow from Traefik to kamal-proxy to your app.
+
+```
+$ docker ps
+CONTAINER ID   IMAGE                                                                     COMMAND                  CREATED              STATUS              PORTS                               NAMES
+3729c50d9d94   registry:4443/app_with_traefik:30482914d55f9ca5e4302dd2d050e424d29d8f74   "/docker-entrypoint.…"   11 seconds ago       Up 10 seconds       80/tcp                              app_with_traefik-web-30482914d55f9ca5e4302dd2d050e424d29d8f74
+3c87e1c649e3   basecamp/kamal-proxy:v0.4.0                                               "kamal-proxy run"        12 seconds ago       Up 11 seconds       80/tcp, 443/tcp                     kamal-proxy
+609a18d8ecd7   traefik:v2.10                                                             "/entrypoint.sh --pr…"   About a minute ago   Up About a minute   0.0.0.0:80->80/tcp, :::80->80/tcp   traefik
+```
+
+## Switching on a host already running kamal-proxy
+
+If you are already running kamal-proxy, you'll need to:
+
+1. Manually run the `kamal proxy boot_config set` command from the deploy hook
+2. Run `kamal proxy reboot` to pick up those boot config changes
+3. Run `kamal accessory boot traefik` to start Traefik