Update README.md

docker/rpmbuild/README.md

@@ -8,6 +8,7 @@ For more information on docker and containerization technologies, refer to [offi
 
 - `latest`, `8` [*(Dockerfile)*][2]
 - `7` [*(Dockerfile)*][3]
+- `6` [*(Dockerfile)*][7]
 
 ### Reference
 
@@ -27,6 +28,12 @@ The `builder` user (UID 1000) is a member of users and wheel, and has password-l
 
 ## Pull the image
 
+For CentOS 6
+
+```bash
+$ docker pull teddysun/rpmbuild:6
+```
+
 For CentOS 7
 
 ```bash
@@ -43,6 +50,13 @@ It can be found at [Docker Hub][6].
 
 ## Start a container
 
+There is an example to start a container for CentOS 6 like below:
+
+```bash
+$ mkdir -m 777 -p /opt/builder6
+$ docker run -it --rm -h buildbot --name rpmbuild6 -v /opt/builder6:/home/builder/rpmbuild teddysun/rpmbuild:6
+```
+
 There is an example to start a container for CentOS 7 like below:
 
 ```bash
@@ -62,4 +76,5 @@ $ docker run -it --rm -h buildbot --name rpmbuild8 -v /opt/builder8:/home/builde
 [3]: https://github.com/teddysun/across/blob/master/docker/rpmbuild/Dockerfile.centos7
 [4]: https://github.com/docker-library/official-images#architectures-other-than-amd64
 [5]: https://docs.docker.com/install/
-[6]: https://hub.docker.com/r/teddysun/rpmbuild/
+[6]: https://hub.docker.com/r/teddysun/rpmbuild/
+[7]: https://github.com/teddysun/across/blob/master/docker/rpmbuild/Dockerfile.centos6