docker: Support docker swarm secrets. (#3977)

Fixes #3896

docs/docker/README.md

@@ -1,11 +1,9 @@
 # Minio Docker Quickstart Guide [![Slack](https://slack.minio.io/slack?type=svg)](https://slack.minio.io) [![Go Report Card](https://goreportcard.com/badge/minio/minio)](https://goreportcard.com/report/minio/minio) [![Docker Pulls](https://img.shields.io/docker/pulls/minio/minio.svg?maxAge=604800)](https://hub.docker.com/r/minio/minio/) [![codecov](https://codecov.io/gh/minio/minio/branch/master/graph/badge.svg)](https://codecov.io/gh/minio/minio)
 
 ## Prerequisites
-
 Docker installed on your machine. Download the relevant installer from [here](https://www.docker.com/community-edition#/download).
 
 ## Run Standalone Minio on Docker.
-
 Minio needs a persistent volume to store configuration and application data. However, for testing purposes, you can launch Minio by simply passing a directory (`/export` in the example below). This directory gets created in the container filesystem at the time of container start. But all the data is lost after container exits.
 
 ```sh
@@ -15,7 +13,6 @@ docker run -p 9000:9000 minio/minio server /export
 To create a Minio container with persistent storage, you need to map local persistent directories from the host OS to virtual config `~/.minio` and export `/export` directories. To do this, run the below commands
 
 #### GNU/Linux and macOS
-
 ```sh
 docker run -p 9000:9000 --name minio1 \
   -v /mnt/export/minio1:/export \
@@ -24,7 +21,6 @@ docker run -p 9000:9000 --name minio1 \
 ```
 
 #### Windows
-
 ```sh
 docker run -p 9000:9000 --name minio1 \
   -v D:\export\minio1:/export \
@@ -33,7 +29,6 @@ docker run -p 9000:9000 --name minio1 \
 ```
 
 ## Run Distributed Minio on Docker
-
 Distributed Minio can be deployed via [Docker Compose](https://docs.docker.com/compose/overview/) or [Swarm mode](https://docs.docker.com/engine/swarm/). The major difference between these two being, Docker Compose creates a single host, multi-container deployment, while Swarm mode creates a multi-host, multi-container deployment.
 
 This means Docker Compose lets you quickly get started with Distributed Minio on your computer - ideal for development, testing, staging environments. While deploying Distributed Minio on Swarm offers a more robust, production level deployment.
@@ -44,11 +39,9 @@ This means Docker Compose lets you quickly get started with Distributed Minio on
 ## Minio Docker Tips
 
 ### Minio Custom Access and Secret Keys
-
 To override Minio's auto-generated keys, you may pass secret and access keys explicitly as environment variables. Minio server also allows regular strings as access and secret keys.
 
 #### GNU/Linux and macOS
-
 ```sh
 docker run -p 9000:9000 --name minio1 \
   -e "MINIO_ACCESS_KEY=AKIAIOSFODNN7EXAMPLE" \
@@ -59,8 +52,7 @@ docker run -p 9000:9000 --name minio1 \
 ```
 
 #### Windows
-
-```sh
+```powershell
 docker run -p 9000:9000 --name minio1 \
   -e "MINIO_ACCESS_KEY=AKIAIOSFODNN7EXAMPLE" \
   -e "MINIO_SECRET_KEY=wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY" \
@@ -68,8 +60,23 @@ docker run -p 9000:9000 --name minio1 \
   -v D:\export\minio1-config:/root/.minio \
   minio/minio server /export
 ```
-### Retrieving Container ID
 
+### Minio Custom Access and Secret Keys using Docker secrets
+To override Minio's auto-generated keys, you may pass secret and access keys explicitly by creating access and secret keys as [Docker secrets](https://docs.docker.com/engine/swarm/secrets/). Minio server also allows regular strings as access and secret keys.
+
+```
+echo "AKIAIOSFODNN7EXAMPLE" | docker secret create access_key -
+echo "wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY" | docker secret create secret_key -
+```
+
+Create a Minio service using `docker service` to read from Docker secrets.
+```
+docker service create --name="minio-service" --secret="access_key" --secret="secret_key" minio/minio server /export
+```
+
+Read more about `docker service` [here](https://docs.docker.com/engine/swarm/how-swarm-mode-works/services/)
+
+### Retrieving Container ID
 To use Docker commands on a specific container, you need to know the `Container ID` for that container. To get the `Container ID`, run
 
 ```sh
@@ -79,7 +86,6 @@ docker ps -a
 `-a` flag makes sure you get all the containers (Created, Running, Exited). Then identify the `Container ID` from the output.
 
 ### Starting and Stopping Containers
-
 To start a stopped container, you can use the [`docker start`](https://docs.docker.com/engine/reference/commandline/start/) command.
 
 ```sh
@@ -87,13 +93,11 @@ docker start <container_id>
 ```
 
 To stop a running container, you can use the [`docker stop`](https://docs.docker.com/engine/reference/commandline/stop/) command.
-
 ```sh
 docker stop <container_id>
 ```
 
 ### Minio container logs
-
 To access Minio logs, you can use the [`docker logs`](https://docs.docker.com/engine/reference/commandline/logs/) command.
 
 ```sh
@@ -101,7 +105,6 @@ docker logs <container_id>
 ```
 
 ### Monitor Minio Docker Container
-
 To monitor the resources used by Minio container, you can use the [`docker stats`](https://docs.docker.com/engine/reference/commandline/stats/) command.
 
 ```sh