Fix missing 'locales' package in base image

`/bin/sh: 1: locale-gen: not found`

Dockerfile

@@ -5,7 +5,6 @@ FROM ubuntu:16.04
 ENV DEBIAN_FRONTEND noninteractive
 
 # Set the locale
-RUN locale-gen en_US.UTF-8  
 ENV LANG en_US.UTF-8  
 ENV LANGUAGE en_US:en  
 ENV LC_ALL en_US.UTF-8 
@@ -13,6 +12,7 @@ ENV LC_ALL en_US.UTF-8
 RUN apt-get update && apt-get install -y software-properties-common
 RUN add-apt-repository ppa:gns3/ppa
 RUN apt-get update && apt-get install -y \
+    locales \
     python3-pip \
     python3-dev \ 
     qemu-system-x86 \
@@ -21,6 +21,8 @@ RUN apt-get update && apt-get install -y \
     libvirt-bin \
     x11vnc 
 
+RUN locale-gen en_US.UTF-8
+
 # Install uninstall to install dependencies
 RUN apt-get install -y vpcs ubridge