minor adjustments to Dockerfile

Dockerfile

@@ -2,13 +2,18 @@ FROM ubuntu:16.04
 
 MAINTAINER z4yx <z4yx@users.noreply.github.com>
 
-# build with docker build --build-arg PETA_VERSION=2018.1 --build-arg PETA_RUN_FILE=petalinux-v2018.1-final-installer.run -t petalinux:2018.1 .
+# Build with:
+# docker build \
+#   --build-arg PETA_VERSION=2018.1 \
+#   --build-arg PETA_RUN_FILE=petalinux-v2018.1-final-installer.run \
+#   -t petalinux:2018.1 .
 
-ARG UBUNTU_MIRROR=mirror.tuna.tsinghua.edu.cn
+ENV DEBIAN_FRONTEND=noninteractive
 
 #install dependences:
-RUN sed -i.bak s/archive.ubuntu.com/${UBUNTU_MIRROR}/g /etc/apt/sources.list && \
-  dpkg --add-architecture i386 && apt-get update &&  DEBIAN_FRONTEND=noninteractive apt-get install -y -q \
+RUN dpkg --add-architecture i386 && \
+    apt-get update && \
+    apt-get install -y -q \
         build-essential \
         sudo \
         tofrodos \
@@ -60,14 +65,14 @@ ARG PETA_RUN_FILE
 
 RUN locale-gen en_US.UTF-8 && update-locale
 
-#make a Vivado user
+# Create a Vivado user
 RUN adduser --disabled-password --gecos '' vivado && \
     usermod -aG sudo vivado && \
     echo "vivado ALL=(ALL) NOPASSWD: ALL" >> /etc/sudoers
 
 COPY accept-eula.sh ${PETA_RUN_FILE} /
 
-# run the install
+# Run the installer
 RUN chmod a+rx /${PETA_RUN_FILE} && \
     chmod a+rx /accept-eula.sh && \
     mkdir -p /opt/Xilinx && \
@@ -76,7 +81,7 @@ RUN chmod a+rx /${PETA_RUN_FILE} && \
     sudo -u vivado -i /accept-eula.sh /${PETA_RUN_FILE} /opt/Xilinx/petalinux && \
     rm -f /${PETA_RUN_FILE} /accept-eula.sh 
 
-# make /bin/sh symlink to bash instead of dash:
+# Make /bin/sh symlink to bash instead of dash:
 RUN echo "dash dash/sh boolean false" | debconf-set-selections
 RUN DEBIAN_FRONTEND=noninteractive dpkg-reconfigure dash
 
@@ -86,6 +91,6 @@ ENV LANG en_US.UTF-8
 RUN mkdir /home/vivado/project
 WORKDIR /home/vivado/project
 
-#add vivado tools to path
+# Add vivado tools to path
 RUN echo "source /opt/Xilinx/petalinux/settings.sh" >> /home/vivado/.bashrc