docker: Move the docker document and compose to a separate dir. (#3135)

docs/docker/README.md

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+# Minio Docker Quickstart Guide [![Gitter](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/minio/minio?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+
+## 1. Test Minio on Docker.
+Minio generates new access and secret keys each time you run this command. Container state is lost after you end this session. This mode is only intended for testing purpose.
+
+```sh
+
+docker run -p 9000:9000 minio/minio server /export
+
+```
+
+## 2. Run Minio Standalone on Docker.
+
+Minio container requires a persistent volume to store configuration and application data. Following command maps local persistent directories from the host OS to virtual config `~/.minio` and export `/export` directories.
+
+```sh
+
+docker run -p 9000:9000 --name minio1 \
+  -v /mnt/export/minio1:/export \
+  -v /mnt/config/minio1:/root/.minio \
+  minio/minio server /export
+
+```
+
+## 3. Run Minio Standalone on Docker with 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.
+
+```sh
+
+docker run -p 9000:9000 --name minio1 \
+  -e "MINIO_ACCESS_KEY=AKIAIOSFODNN7EXAMPLE" \
+  -e "MINIO_SECRET_KEY=wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY" \
+  -v /mnt/export/minio1:/export \
+  -v /mnt/config/minio1:/root/.minio \
+  minio/minio server /export
+
+```
+
+## 4. Test Distributed Minio on Docker
+
+To run 4 minio servers inside different docker containers using [docker-compose](https://docs.docker.com/compose/). Please download [docker-compose.yml](./docker-compose.yml) to your current working directory.
+
+### Run `docker-compose`
+
+```sh
+docker-compose pull
+docker-compose up
+```
+
+Each instance is accessible on the host at ports 9001 through 9004, proceed to access the Web browser at http://localhost:9001/