Add a xenial/snapcraft environment

Signed-off-by: Konstantin Pavlov <thresh@videolan.org>

vlc-ubuntu-xenial/Dockerfile

+FROM videolan-base-xenial:latest
+
+# Don't clean the package lists cache, snapcraft needs it.
+RUN apt-get update && \
+    apt-get install -y git-core make build-essential snapcraft locales && \
+    apt-get clean -y && \
+    locale-gen en_US.UTF-8 && \
+    /usr/sbin/update-locale LANG=en_US.UTF-8
+
+ENV LANG=en_US.UTF-8
+

vlc-ubuntu-xenial/Makefile

+.PHONY: build
+
+build:
+	docker build -t jenkins-vlc-xenial:latest .