feat: add docker external plugin

Signed-off-by: Jose Diaz-Gonzalez <email@josediazgonzalez.com>

Author: josegonzalez

content/explugins/docker.md

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++++
+title = "docker"
+description = "*example* - enables reading zone data from the Docker Daemon"
+weight = 10
+tags = [  "plugin" , "docker" ]
+categories = [ "plugin", "external" ]
+date = "2025-12-18T18:39:00+04:00"
+repo = "https://github.com/dokku/coredns-docker"
+home = "https://github.com/dokku/coredns-docker/blob/master/README.md"
++++
+
+## Description
+
+The docker plugin serves DNS records for containers running on the local Docker daemon. It follows the Docker event stream, picking up changes whenever something happens to a container - whether it gets created, started, deleted, or restarted.
+
+The plugin resolves container names, network aliases, DNS names, and SRV records to their respective container IP addresses within a specified network.
+
+SRV records can be defined using container labels with the prefix `[LABEL_PREFIX].srv.`, followed by the protocol and service name. For example, with the default prefix, a label `com.dokku.coredns-docker.srv._tcp._http=80` will create an SRV record for `_http._tcp.container-name.domain` pointing to the container's IP on port 80.
+
+If no labels with the specified prefix are found, the plugin falls back to using the container's exposed ports (`NetworkSettings.Ports`).
+
+- For a port mapping like `80/tcp`, it generates an SRV record for `_tcp._tcp.container-name.domain`.
+- For a port mapping without a protocol like `80`, it generates SRV records for both `_tcp._tcp` and `_udp._udp`.
+
+## Compilation
+
+It will require you to use `go get` or as a dependency on [plugin.cfg](https://github.com/coredns/coredns/blob/master/plugin.cfg).
+
+A simple way to consume this plugin, is by adding the following on [plugin.cfg](https://github.com/coredns/coredns/blob/master/plugin.cfg), and recompile it as [detailed on coredns.io](https://coredns.io/2017/07/25/compile-time-enabling-or-disabling-plugins/#build-with-compile-time-configuration-file).
+
+```text
+docker:github.com/dokku/coredns-docker
+```
+
+After this you can compile coredns by running:
+
+```bash
+make
+```
+
+## Syntax
+
+```text
+docker [DOMAIN] {
+    ttl DURATION
+    label_prefix PREFIX
+    max_backoff DURATION
+    networks NETWORK...
+}
+```
+
+- `DOMAIN` is the domain for which the plugin will respond. Defaults to `docker.`.
+
+- `ttl` allows you to set a custom TTL for responses. **DURATION** defaults to `30 seconds`. The minimum TTL allowed is `0` seconds, and the maximum is capped at `3600` seconds. Setting TTL to 0 will prevent records from being cached. The unit for the value is seconds.
+
+- `label_prefix` allows you to set a custom prefix for SRV record labels. **PREFIX** defaults to `com.dokku.coredns-docker`.
+
+- `max_backoff` allows you to set a maximum backoff duration for the Docker event loop reconnection logic. **DURATION** defaults to `60s`.
+
+- `networks` allows you to specify a list of Docker networks to monitor. If specified, containers not on one of these networks will be ignored.
+
+## Metrics
+
+If monitoring is enabled (via the *prometheus* directive) the following metric is exported:
+
+- `coredns_docker_success_requests_total{server}` - Counter of DNS requests handled successfully.
+- `coredns_docker_failed_requests_total{server}` - Counter of DNS requests failed.
+
+The `server` label indicated which server handled the request.
+
+## Ready
+
+This plugin reports readiness to the ready plugin. It will be ready only when it has successfully connected to the Docker daemon.
+
+## Examples
+
+Enable docker with and resolve all containers with `.docker.` as the suffix.
+
+```text
+docker:1053 {
+    docker docker.
+    cache 30
+}
+```
+
+You can see the [Corefile.example](./Corefile.example) for a full Corefile example.
+
+## Usage Example
+
+### A record
+
+```shell
+dig web.docker @127.0.0.1 -p 1053    
+
+; <<>> DiG 9.18.1-1ubuntu1.2-Ubuntu <<>> web.docker @127.0.0.1 -p 1053
+;; global options: +cmd
+;; Got answer:
+;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 54986
+;; flags: qr aa rd; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0
+
+;; QUESTION SECTION:
+;web.docker.  IN A
+
+;; ANSWER SECTION:
+web.docker. 30 IN A 172.17.0.2
+
+;; Query time: 4 msec
+;; SERVER: 127.0.0.1#1053(127.0.0.1) (UDP)
+```
+
+### SRV record
+
+```shell
+dig _http._tcp.web.docker @127.0.0.1 -p 1053 SRV
+
+; <<>> DiG 9.18.1-1ubuntu1.2-Ubuntu <<>> _http._tcp.web.docker @127.0.0.1 -p 1053 SRV
+;; global options: +cmd
+;; Got answer:
+;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 49945
+;; flags: qr aa rd; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0
+
+;; QUESTION SECTION:
+;_http._tcp.web.docker.  IN SRV
+
+;; ANSWER SECTION:
+_http._tcp.web.docker. 30 IN SRV 10 10 80 web.docker.
+
+;; Query time: 0 msec
+;; SERVER: 127.0.0.1#1053(127.0.0.1) (UDP)
+```