fix 8fe0e155d3: bump traefik to version 2.7

the :2.6.3 tag is apparently only available for arm arch, it's probably
best not to base the decision to bump versions on a new release
notification. instead, availability of the new version for our default
arch (amd64) should always be checked:
    podman run -it --rm docker.io/library/traefik:2.6.3
    Trying to pull docker.io/library/traefik:2.6.3...
    Error: choosing an image from manifest list docker://traefik:2.6.3: no image found in manifest list for architecture amd64, variant "", OS linux

    ~ took 2s
    (╯°□°）╯︵ ┻━┻ 125 🔥  podman run -it --rm docker.io/library/traefik:2.7
    Trying to pull docker.io/library/traefik:2.7...
    Getting image source signatures
    Copying blob 491249faa733 done

docker-compose.yml

@@ -1,7 +1,7 @@
 version: '3'
 services:
   reverse-proxy:
-    image: traefik:2.6.3
+    image: traefik:2.7
     command: --api=false --api.dashboard=false --providers.docker
     networks:
     - internal-nw