docker: Add README from old Docker repo (fixes #4868) (#4869)

With slight modifications

README-Docker.md

@@ -0,0 +1,43 @@
+# Docker Container for Syncthing
+
+Use the Dockerfile in this repo, or pull the `syncthing/syncthing` image
+from Docker Hub. Use volumes to have the synchronized files available on the
+host.
+
+The exposed volumes are by default:
+
+    /var/syncthing/config   - the configuration and index directory into the Container
+    /var/syncthing          - the default sync folder into the Container
+
+You can add more folders and map them as you prefer.
+
+Note that Syncthing runs as UID 1000 in the container. This UID must have
+permission to read and modify the files in the containers.
+
+Example usage:
+
+```
+$ docker pull syncthing/syncthing
+$ docker run -p 8384:8384 -p 22000:22000 \
+    -v /wherever/st-cfg:/var/syncthing/config \
+    -v /wherever/st-sync:/var/syncthing \
+    syncthing/syncthing:latest
+```
+
+Note that local device discovery will not work with the above command resulting
+in poor local transfer rates if local device addresses are not manually
+configured.
+
+To allow local discovery, the docker host network can be used instead:
+
+```
+$ docker pull syncthing/syncthing
+$ docker run --network=host \
+    -v /wherever/st-cfg:/var/syncthing/config \
+    -v /wherever/st-sync:/var/syncthing \
+    syncthing/syncthing:latest
+```
+
+Be aware that syncthing alone is now in control of what interfaces and ports it
+listens on. You can edit the syncthing configuration to change the defaults if
+there are conflicts.