Update Docker Hub README and keep it in sync with this repository.

This commit
* adds a new docs page (`dockerhub.md`) that contains Docker Hub README
* updates the default config that gets backed into the docker image
* updates CI with a new workflow job that keeps Docker Hub README in
  sync with the contents of the `docs/dockerhub.md` file

Co-authored-by: CrazyMax <github@crazymax.dev>
Signed-off-by: Milos Gajdos <milosthegajdos@gmail.com>

cmd/registry/config-dev.yml

@@ -4,20 +4,6 @@ log:
   fields:
     service: registry
     environment: development
-  hooks:
-    - type: mail
-      disabled: true
-      levels:
-        - panic
-      options:
-        smtp:
-          addr: mail.example.com:25
-          username: mailuser
-          password: password
-          insecure: true
-        from: sender@example.com
-        to:
-          - errors@example.com
 storage:
     delete:
       enabled: true
@@ -37,33 +23,6 @@ http:
             path: /metrics
     headers:
         X-Content-Type-Options: [nosniff]
-redis:
-  addr: localhost:6379
-  pool:
-    maxidle: 16
-    maxactive: 64
-    idletimeout: 300s
-  dialtimeout: 10ms
-  readtimeout: 10ms
-  writetimeout: 10ms
-notifications:
-    events:
-        includereferences: true
-    endpoints:
-        - name: local-5003
-          url: http://localhost:5003/callback
-          headers:
-             Authorization: [Bearer <an example token>]
-          timeout: 1s
-          threshold: 10
-          backoff: 1s
-          disabled: true
-        - name: local-8083
-          url: http://localhost:8083/callback
-          timeout: 1s
-          threshold: 10
-          backoff: 1s
-          disabled: true
 health:
   storagedriver:
     enabled: true