Added Dockerfile to build ubuntudesktop/gnome-3-26-1604 images

Dockerfile

@@ -0,0 +1,17 @@
+FROM snapcore/snapcraft:stable
+
+RUN apt-get install -q -y --no-install-recommends software-properties-common && \
+  add-apt-repository -yu ppa:ubuntu-desktop/gnome-3-26 && \
+  apt-get update && \
+  apt-get dist-upgrade --yes && \
+  apt-get install --yes \
+  build-essential \
+  libgtk-3-dev \
+  snapcraft \
+  && \
+  apt-get autoclean --yes && \
+  apt-get clean --yes
+
+# Required by click.
+ENV LC_ALL C.UTF-8
+ENV SNAPCRAFT_SETUP_CORE 1