Add step to install software-properties-common (docker#2523)

This is needed for the `lsb_release` command that is used further down the Dockerfile.

Fixes docker#424.

Author: Misty Stanley-Jones

engine/examples/mongodb.md

@@ -63,6 +63,7 @@ a MongoDB repository file for the package manager.
 # Installation:
 # Import MongoDB public GPG key AND create a MongoDB list file
 RUN apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv EA312927
+RUN apt-get install --no-install-recommends software-properties-common
 RUN echo "deb http://repo.mongodb.org/apt/ubuntu $(cat /etc/lsb-release | grep DISTRIB_CODENAME | cut -d= -f2)/mongodb-org/3.2 multiverse" | tee /etc/apt/sources.list.d/mongodb-org-3.2.list
 ```
 
@@ -74,8 +75,9 @@ RUN apt-get update && apt-get install -y mongodb-org
 ```
 
 > **Tip**: You can install a specific version of MongoDB by using a list of required packages with versions, e.g.:
-```dockerfile
-RUN apt-get update && apt-get install -y mongodb-org=3.0.1 mongodb-org-server=3.0.1 mongodb-org-shell=3.0.1 mongodb-org-mongos=3.0.1 mongodb-org-tools=3.0.1
+>
+> ```dockerfile
+> RUN apt-get update && apt-get install -y mongodb-org=3.0.1 mongodb-org-server=3.0.1 mongodb-org-shell=3.0.1 mongodb-org-> mongos=3.0.1 mongodb-org-tools=3.0.1
 ```
 
 MongoDB requires a data directory. Let's create it as the final step of our