add Docker README

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+# Intro
+
+https://hub.docker.com/u/powerdns offers automatic builds of dnsdist, Auth and Recursor, from the pdns.git master branch.
+
+The images are based on Debian Buster (slim).
+
+The Dockerfiles:
+
+* https://github.com/PowerDNS/pdns/blob/master/Dockerfile-auth
+* https://github.com/PowerDNS/pdns/blob/master/Dockerfile-dnsdist
+* https://github.com/PowerDNS/pdns/blob/master/Dockerfile-recursor
+
+Other data involved in the Docker build process can be found at https://github.com/PowerDNS/pdns/tree/master/dockerdata
+
+# Usage
+
+The images are ready to run with limited functionality.
+At container startup, the startup.py wrapper (from the dockerdata directory linked above) checks for `PDNS_RECURSOR_API_KEY` / `PDNS_AUTH_API_KEY` / `DNSDIST_API_KEY` environment variables for the product you are running.
+If such a variable is found, `/etc/powerdns-api.conf` or `/etc/dnsdist-api.conf` is written, enabling the webserver in all products, and the dnsdist console.
+For the dnsdist console, make sure that your API key is in a format suitable for the console (use `makeKey()`).
+
+The default configs shipped in the image (see dockerdata above) parse all files in `/etc/powerdns/pdns.d` / `/etc/powerdns/recursor.d` / `/etc/dnsdist/conf.d`.
+The image also ships a symlink to the API config file inside those `.d` dirs.
+For Auth and Recursor, extra configuration can be passed or the command line, or via a volume mount into `/etc/powerdns` or the `.d` dir.
+For dnsdist, only the volume mount is applicable.
+
+If you want to volume mount a config, but also take the keys from the environment, please take care to include the same `X-api.conf` symlink in your `.d` directory.
+
+# Auth and databases
+
+The default auth config uses SQLite3 in `/var/lib/powerdns/pdns.sqlite3`.
+We suggest providing that file via a volume.
+For other databases, either pass config overrides on the command line, or mount a config file.
+
+## LMDB
+
+When using the LMDB backend with the auth image, please make sure that any containers that might access the database volume do not have overlapping PIDs - otherwise you will get locking problems and possibly resulting corruption.
+In a plain Docker or Compose setup, this can be done by using the host PID namespace.

diff --git a/README.md b/README.md
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@@ -15,6 +15,12 @@ https://github.com/PowerDNS/pdns/issues
 
 But please check if the issue is already reported there first.
 
+DOCKER
+------
+
+This README is mirrored from GitHub to dockerhub.
+For information about our Docker images, please refer to https://github.com/PowerDNS/pdns/blob/master/Docker-README.md
+
 SOURCE CODE / GIT
 -----------------
 Source code is available on GitHub: