doc/manual: add documentation for non-root container images

doc/manual/source/installation/installing-docker.md

@@ -57,3 +57,21 @@ $ nix build ./\#hydraJobs.dockerImage.x86_64-linux
 $ docker load -i ./result/image.tar.gz
 $ docker run -ti nix:2.5pre20211105
 ```
+
+# Docker image with non-root Nix
+
+If you would like to run Nix in a container under a user other than `root`,
+you can build an image with a non-root single-user installation of Nix
+by specifying the `uid`, `gid`, `uname`, and `gname` arguments to `docker.nix`:
+
+```console
+$ nix build --file docker.nix \
+    --arg uid 1000 \
+    --arg gid 1000 \
+    --argstr uname user \
+    --argstr gname user \
+    --argstr name nix-user \
+    --out-link nix-user.tar.gz
+$ docker load -i nix-user.tar.gz
+$ docker run -ti nix-user
+```