Docker image workflow (publish on ghcr.io) (#14230)

doc/docker_server.md

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+# Docker Server
+
+We provide Minetest server Docker images using the GitHub container registry.
+
+Images are built on each commit and available using the following tag scheme:
+
+* `ghcr.io/minetest/minetest:master` (latest build)
+* `ghcr.io/minetest/minetest:<tag>` (specific Git tag)
+* `ghcr.io/minetest/minetest:latest` (latest Git tag, which is the stable release)
+
+See [here](https://github.com/minetest/minetest/pkgs/container/minetest) for all available tags.
+
+For a quick test you can easily run:
+
+```shell
+docker run ghcr.io/minetest/minetest:master
+```
+
+To use it in a production environment, you should use volumes bound to the Docker host to persist data and modify the configuration:
+
+```shell
+docker create -v /home/minetest/data/:/var/lib/minetest/ -v /home/minetest/conf/:/etc/minetest/ ghcr.io/minetest/minetest:master
+```
+
+You may also want to use [Docker Compose](https://docs.docker.com/compose):
+
+```yaml
+---
+version: "2"
+services:
+  minetest_server:
+    image: ghcr.io/minetest/minetest:master
+    restart: always
+    networks:
+      - default
+    volumes:
+      - /home/minetest/data/:/var/lib/minetest/
+      - /home/minetest/conf/:/etc/minetest/
+    ports:
+      - "30000:30000/udp"
+      - "127.0.0.1:30000:30000/tcp"
+```
+
+Data will be written to `/home/minetest/data` on the host, and configuration will be read from `/home/minetest/conf/minetest.conf`.
+
+**Note:** If you don't understand the previous commands please read the [official Docker documentation](https://docs.docker.com) before use.