docs: mention deSEC feature in readme, reverse-proxy, local-instance and QA docs

Agent-Logs-Url: https://github.com/nextcloud/all-in-one/sessions/987af94f-dbff-4efd-968c-f6993c7c4dc8

Co-authored-by: szaimen <42591237+szaimen@users.noreply.github.com>

readme.md

@@ -61,6 +61,7 @@ Included are:
 - [Mail server can be added](https://github.com/nextcloud/all-in-one#mail-server)
 - Nextcloud can be [accessed locally via the domain](https://github.com/nextcloud/all-in-one#how-can-i-access-nextcloud-locally)
 - Can [be installed locally](https://github.com/nextcloud/all-in-one/blob/main/local-instance.md) (if you don't want or cannot make the instance publicly reachable)
+- Free [deSEC](https://desec.io) dynamic-DNS domain (`*.dedyn.io`) can be registered directly from the AIO interface — no external domain needed
 - [IPv6-ready](https://github.com/nextcloud/all-in-one/blob/main/docker-ipv6-support.md)
 - Can be used with [Docker rootless](https://github.com/nextcloud/all-in-one/blob/main/docker-rootless.md) (good for additional security)
 - Runs on all platforms Docker supports (e.g. also on Windows and Macos)
@@ -248,6 +249,9 @@ https://your-domain-that-points-to-this-server.tld:8443
 
 5. If you enable Nextcloud Talk, open port `3478/TCP` and `3478/UDP` in your firewall/router for the Talk (TURN) container.
 
+> [!TIP]
+> Don't have a domain yet? AIO can register a free dynamic-DNS subdomain under `dedyn.io` for you via [deSEC](https://desec.io) — no external setup needed. Look for the **"Don't have a domain? Get a free one from deSEC"** section on the AIO interface when you are asked to enter your domain. AIO will automatically register the domain, keep the DNS record up to date, and enable the [Caddy](https://github.com/nextcloud/all-in-one/tree/main/community-containers/caddy) community container as a reverse proxy as well as the [dnsmasq](https://github.com/nextcloud/all-in-one/tree/main/community-containers/dnsmasq) container for local DNS resolution.
+
 # FAQ
 - [TOC](#faq)
     - [Where can I find additional documentation?](#where-can-i-find-additional-documentation)
@@ -261,6 +265,7 @@ https://your-domain-that-points-to-this-server.tld:8443
     - [Notes on Cloudflare (proxy/tunnel)](#notes-on-cloudflare-proxytunnel)
     - [How to run Nextcloud behind a Cloudflare Tunnel?](#how-to-run-nextcloud-behind-a-cloudflare-tunnel)
     - [How to run Nextcloud via Tailscale?](#how-to-run-nextcloud-via-tailscale)
+    - [How to get a free domain via deSEC?](#how-to-get-a-free-domain-via-desec)
     - [How to get Nextcloud running using the ACME DNS-challenge?](#how-to-get-nextcloud-running-using-the-acme-dns-challenge)
     - [How to run Nextcloud locally? No domain wanted, or wanting intranet access within your LAN.](#how-to-run-nextcloud-locally-no-domain-wanted-or-wanting-intranet-access-within-your-lan)
     - [Can I use an ip-address for Nextcloud instead of a domain?](#can-i-use-an-ip-address-for-nextcloud-instead-of-a-domain)
@@ -410,6 +415,19 @@ Although it does not seems like it is the case but from AIO perspective a Cloudf
 ### How to run Nextcloud via Tailscale?
 For a reverse proxy example guide for Tailscale, see this guide by [@Perseus333](https://github.com/Perseus333): https://github.com/nextcloud/all-in-one/discussions/6817
 
+### How to get a free domain via deSEC?
+[deSEC](https://desec.io) offers free dynamic-DNS subdomains under `dedyn.io`. AIO integrates the registration process directly:
+
+1. Open the AIO interface and expand the **"Don't have a domain? Get a free one from deSEC"** section on the domain-entry page.
+2. Enter your email address and, optionally, a desired subdomain slug (the part before `.dedyn.io`). Leave the slug blank for a random one.
+3. Click **Register free domain via deSEC**. AIO will create a deSEC account, register the subdomain and store the credentials in its configuration.
+
+After successful registration:
+- AIO sets the registered `*.dedyn.io` domain as the Nextcloud domain automatically.
+- The [Caddy](https://github.com/nextcloud/all-in-one/tree/main/community-containers/caddy) community container is enabled as a reverse proxy.
+- The [dnsmasq](https://github.com/nextcloud/all-in-one/tree/main/community-containers/dnsmasq) community container is enabled so that LAN devices can resolve the domain to the server's local IP. Follow the [dnsmasq documentation](https://github.com/nextcloud/all-in-one/tree/main/community-containers/dnsmasq) for the required router change.
+- The mastercontainer keeps the DNS record up to date automatically when your public IP changes.
+
 ### How to get Nextcloud running using the ACME DNS-challenge?
 You can install AIO behind an external reverse proxy where is also documented how to get it running using the ACME DNS-challenge for getting a valid certificate for AIO. See the [reverse proxy documentation](./reverse-proxy.md). (Meant is the `Caddy with ACME DNS-challenge` section). Also see https://github.com/dani-garcia/vaultwarden/wiki/Running-a-private-vaultwarden-instance-with-Let%27s-Encrypt-certs#getting-a-custom-caddy-build for additional docs on this topic.
 
@@ -446,6 +464,8 @@ Now that this is out of the way, the recommended way how to access Nextcloud loc
 - https://dockerlabs.collabnix.com/intermediate/networking/Configuring_DNS.html
 Apart from that there is now a community container that can be added to the AIO stack: https://github.com/nextcloud/all-in-one/tree/main/community-containers/pi-hole
 
+If you registered your domain via the built-in [deSEC integration](#how-to-get-a-free-domain-via-desec), the [dnsmasq](https://github.com/nextcloud/all-in-one/tree/main/community-containers/dnsmasq) community container is automatically enabled and resolves your Nextcloud domain to the server's local IP for LAN clients. You only need to point your router's DHCP DNS server to the AIO host — see the [dnsmasq documentation](https://github.com/nextcloud/all-in-one/tree/main/community-containers/dnsmasq) for details.
+
 ### How to overwrite the local DNS resolution for some domains or add extra hosts to the containers?
 
 For some use cases, you might need to overwrite the local DNS resolution of some domains inside the containers. On Linux, you can do so either by using a local DNS server as described in the section above and add a local DNS entry into the dns server and make your containers use that DNS server.