Update README and Dockerfile

Dockerfile

 FROM centos:6
 
-RUN 	sed -i '/^mirrorlist/s/^/#/;/^#baseurl/{s/#//;s/mirror.centos.org\/centos\/$releasever/vault.centos.org\/6.10/}' /etc/yum.repos.d/*B* ; \
-	yum -y update && yum clean all ; \
-	yum -y groupinstall -y "Development Tools" ; \
-	yum -y install cmake ; \
-	yum -y install epel-release ; \
-	yum -y install cmake3
+RUN sed -i '/^mirrorlist/s/^/#/;/^#baseurl/{s/#//;s/mirror.centos.org\/centos\/$releasever/vault.centos.org\/6.10/}' /etc/yum.repos.d/*B* \
+&& yum -y update && yum clean all \
+&& yum -y groupinstall -y "Development Tools" \
+&& yum -y install cmake \
+&& yum -y install epel-release \
+&& yum -y install cmake3
 
 CMD ["/bin/bash"]
 

README.md

 # dev-el6
 
-To build a CentOS 6 development container for build and unit test
+Docker container image based on official CentOS 6 image with development tools
+
+- gcc version 4.4.7 20120313 (Red Hat 4.4.7-23) (GCC)
+- cmake version 2.8.12.2
+- cmake3 version 3.6.1
+
+## Build image
+
+```
+docker build -t registry.gitlab.com/synchrotron-soleil/control-system/containers/dev-el6 .
+```
+
+If proxy issue
+
+```
+docker build --build-arg http_proxy=http://proxy.synchrotron-soleil.fr:8080 --build-arg https_proxy=http://proxy.synchrotron-soleil.fr:8080 -t registry.gitlab.com/synchrotron-soleil/control-system/containers/dev-el6 .
+```
+
+## Example of usage
+
+```
+$ git clone git@gitlab.synchrotron-soleil.fr:control-system/sandbox/cmake-catch1-hello.git
+...
+$ docker run --rm -it --user $UID -v $(pwd):/data registry.gitlab.com/synchrotron-soleil/control-system/containers/dev-el6
+bash-4.2$ cd /data
+bash-4.2$ cmake3 -H. -Bbuild
+...
+bash-4.2$ cmake3 --build build
+...
+bash-4.2$ build/unit_tests
+...
+bash-4.2$ build/unit_tests -r xml
+...
+bash-4.2$ build/SayHello
+Hello World
+```