add usage instructions for inspec container

README.md

@@ -51,6 +51,31 @@ InSpec requires Ruby ( >1.9 ).
 gem install inspec
 ```
 
+### Usage via Docker
+
+```
+docker pull chef/inspec
+# opens a shell within the container
+docker run -it --rm chef/inspec shell
+```
+
+In order to be able to connect to a remote host via key files, you need to mount the local directory into the container:
+
+```
+$ ls -1
+vagrant
+test.rb
+
+
+$ docker pull chef/inspec
+# The container uses /share as work directory, -v mounts the current directory to /share
+$ docker run -it --rm -v $(pwd):/share chef/inspec exec test.rb -t ssh://root@192.168.64.2:11022 -i vagrant
+..
+
+Finished in 0.04321 seconds (files took 0.54917 seconds to load)
+2 examples, 0 failures
+```
+
 ### Install it from source
 
 That requires [bundler](http://bundler.io/):