Add build/usage info

README.md

@@ -81,3 +81,15 @@ Zend OPcache
 zip
 zlib
 ```
+
+### Build:
+
+```
+docker build --rm -t composer-and-node-ci:latest .
+```
+
+### Basic usage:
+
+```
+docker run --rm --interactive --tty --volume $PWD:/app  composer-and-node-ci  <command>
+```