Move rootless notes to a variant stub

Also, add an explicit note about how to switch the UID/GID and drop the note about 19.03 (long since EOL).

docker/content.md

@@ -32,7 +32,7 @@ Inside the directory specified by `DOCKER_TLS_CERTDIR`, the entrypoint scripts w
 
 In order to make use of this functionality from a "client" container, at least the `client` subdirectory of the `$DOCKER_TLS_CERTDIR` directory needs to be shared (as illustrated in the following examples).
 
-To disable this image behavior, simply override the container command or entrypoint to run `dockerd` directly (`... docker:dind dockerd ...` or `... --entrypoint dockerd docker:dind ...`).
+To disable this image behavior, simply override the container command or entrypoint to run `dockerd` directly (`... %%IMAGE%%:dind dockerd ...` or `... --entrypoint dockerd %%IMAGE%%:dind ...`).
 
 ## Start a daemon instance
 
@@ -205,25 +205,6 @@ $ docker run --privileged --name some-docker -d \
 
 Some of these will not be supported based on the settings on the host's `dockerd`, such as `--ulimit nofile=-1`, giving errors that look like `error setting rlimit type 7: operation not permitted`, and some may inherit sane values from the host `dockerd` instance or may not apply for your usage of Docker-in-Docker (for example, you likely want to set `--oom-score-adj` to a value that's higher than `dockerd` on the host so that your Docker-in-Docker instance is killed before the host Docker instance is).
 
-## Rootless
-
-For more information about using the experimental "rootless" image variants, see [docker-library/docker#174](https://github.com/docker-library/docker/pull/174).
-
-**Note:** just like the regular `dind` images, `--privileged` is required for Docker-in-Docker to function properly ([docker-library/docker#151](https://github.com/docker-library/docker/issues/151#issuecomment-483185972) & [docker-library/docker#281](https://github.com/docker-library/docker/issues/281#issuecomment-744766015)). For `19.03.x` rootless images, an argument of `--experimental` is required for `dockerd` ([docker/docker#40759](https://github.com/docker/docker/pull/40759)).
-
-Basic example usage:
-
-```console
-$ docker run -d --name some-docker --privileged docker:dind-rootless
-$ docker logs --tail=3 some-docker # to verify the daemon has finished generating TLS certificates and is listening successfully
-time="xxx" level=info msg="Daemon has completed initialization"
-time="xxx" level=info msg="API listen on /run/user/1000/docker.sock"
-time="xxx" level=info msg="API listen on [::]:2376"
-$ docker exec -it some-docker docker-entrypoint.sh sh # using "docker-entrypoint.sh" which auto-sets "DOCKER_HOST" appropriately
-/ $ docker info --format '{{ json .SecurityOptions }}'
-["name=seccomp,profile=default","name=rootless"]
-```
-
 ## Where to Store Data
 
 Important note: There are several ways to store data used by applications that run in Docker containers. We encourage users of the `%%REPO%%` images to familiarize themselves with the options available, including:

docker/variant-rootless.md

@@ -0,0 +1,30 @@
+## `%%IMAGE%%:<version>-rootless`
+
+For more information about using the experimental "rootless" image variants, see [docker-library/docker#174](https://github.com/docker-library/docker/pull/174).
+
+**Note:** just like the regular `dind` images, `--privileged` is required for Docker-in-Docker to function properly ([docker-library/docker#151](https://github.com/docker-library/docker/issues/151#issuecomment-483185972) & [docker-library/docker#281](https://github.com/docker-library/docker/issues/281#issuecomment-744766015)), which is a security issue that needs to be treated appropriately.
+
+Basic example usage:
+
+```console
+$ docker run -d --name some-docker --privileged %%IMAGE%%:dind-rootless
+$ docker logs --tail=3 some-docker # to verify the daemon has finished generating TLS certificates and is listening successfully
+time="xxx" level=info msg="Daemon has completed initialization"
+time="xxx" level=info msg="API listen on /run/user/1000/docker.sock"
+time="xxx" level=info msg="API listen on [::]:2376"
+$ docker exec -it some-docker docker-entrypoint.sh sh # using "docker-entrypoint.sh" which auto-sets "DOCKER_HOST" appropriately
+/ $ docker info --format '{{ json .SecurityOptions }}'
+["name=seccomp,profile=default","name=rootless"]
+```
+
+To run with a different UID/GID than the one baked into the image, modify `/etc/passwd`, `/etc/group`, and filesystem permissions (especially for the `rootless` user's home directory) as appropriate; for example:
+
+```dockerfile
+FROM %%IMAGE%%:dind-rootless
+USER root
+RUN set -eux; \
+	sed -i -e 's/^rootless:1000:1000:/rootless:1234:5678:/' /etc/passwd; \
+	sed -i -e 's/^rootless:1000:/:5678:/' /etc/group; \
+	chown -R rootless ~rootless
+USER rootless
+```