Add Ubuntu 16.04 and GCC-6 variant

ubuntu1604-gcc6/Dockerfile

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+FROM ubuntu:16.04
+
+ENV LC_ALL=C DEBIAN_FRONTEND=noninteractive
+
+RUN apt-get update && \
+  apt-get install apt-utils software-properties-common -y && \
+  add-apt-repository -y ppa:ubuntu-toolchain-r/test && \
+  apt-get update && \
+  apt-get dist-upgrade -y && \
+  apt-get install -y \
+    ccache \
+    gcc-6 \
+    g++-6 \
+    git \
+    curl \
+    python-pip \
+    python3-pip \
+    libfontconfig1
+
+RUN update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-6 50 && \
+    update-alternatives --install /usr/bin/c++ c++ /usr/bin/g++-6 50 && \
+    update-alternatives --install /usr/bin/cpp cpp /usr/bin/gcc-6 50 && \
+    update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-6 50 && \
+    update-alternatives --install /usr/bin/cc cc /usr/bin/gcc-6 50
+
+RUN addgroup --gid 1000 iojs && \
+  adduser --gid 1000 --uid 1000 --disabled-password --home /home/iojs --shell /bin/bash --gecos iojs iojs
+
+VOLUME /home/iojs/.ccache /home/iojs/workspace
+
+ENV PATH=/usr/lib/ccache:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin \
+  USER=iojs \
+  JOBS=2 \
+  SHELL=/bin/bash \
+  HOME=/home/iojs \
+  NODE_COMMON_PIPE=/home/iojs/test.pipe \
+  NODE_TEST_DIR=/home/iojs/tmp \
+  OSTYPE=linux-gnu \
+  OSVARIANT=docker \
+  DESTCPU=x64 \
+  ARCH=x64
+
+WORKDIR /home/iojs
+
+USER iojs:iojs

ubuntu1604/Dockerfile

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+FROM ubuntu:16.04
+
+ENV LC_ALL=C DEBIAN_FRONTEND=noninteractive
+
+RUN apt-get update && \
+  apt-get install apt-utils -y && \
+  apt-get dist-upgrade -y && \
+  apt-get install -y \
+    ccache \
+    g++ \
+    gcc \
+    git \
+    curl \
+    python-pip \
+    python3-pip \
+    libfontconfig1
+
+RUN addgroup --gid 1000 iojs && \
+  adduser --gid 1000 --uid 1000 --disabled-password --home /home/iojs --shell /bin/bash --gecos iojs iojs
+
+VOLUME /home/iojs/.ccache /home/iojs/workspace
+
+ENV PATH=/usr/lib/ccache:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin \
+  USER=iojs \
+  JOBS=2 \
+  SHELL=/bin/bash \
+  HOME=/home/iojs \
+  NODE_COMMON_PIPE=/home/iojs/test.pipe \
+  NODE_TEST_DIR=/home/iojs/tmp \
+  OSTYPE=linux-gnu \
+  OSVARIANT=docker \
+  DESTCPU=x64 \
+  ARCH=x64
+
+WORKDIR /home/iojs
+
+USER iojs:iojs