Set the LANG environment variable to en_US.UTF-8 in the zanata-server Dockerfile

[andyrooster]

Set the LANG environment variable to en_US.UTF-8 in the zanata-server Dockerfile.

Without the locale set, when uploading UTF-8 documents containing foreign characters these aren't stored correctly in the database.

For example, upload a file containing the word "Français" and in the app the translation source will be displayed as "Fran��ais"

[zubot]

This change is 

[andyrooster]

@definite I'm not sure if you are the correct person for this but as the top contributor, could this be reviewed/merged?