ubuntu-16.04: add git-lfs via packagecloud apt repo

Install gpg key from wget https://packagecloud.io/github/git-lfs/gpgkey

Install apt-transport-https as packagecloud is https only

Add packagecloud apt repo, as excerpted from install script:
https://packagecloud.io/github/git-lfs/install#bash-deb
(e.g. https://packagecloud.io/install/repositories/github/git-lfs/config_file.list?os=ubuntu&dist=xenial&source=script)

Signed-off-by: Tim Orling <timothy.t.orling@linux.intel.com>

dockerfiles/ubuntu/ubuntu-16.04/ubuntu-16.04-base/Dockerfile

@@ -16,7 +16,12 @@
 
 FROM ubuntu:16.04
 
-RUN apt-get update && \
+# apt-key is using fingerprint from https://packagecloud.io/github/git-lfs/gpgkey
+# wget -O - https://packagecloud.io/github/git-lfs/gpgkey | gpg --with-fingerprint
+RUN apt-get update && apt-get install -y apt-transport-https && \
+    echo "deb https://packagecloud.io/github/git-lfs/ubuntu/ xenial main" >> /etc/apt/sources.list && \
+    apt-key adv --keyserver hkp://p80.pool.sks-keyservers.net:80 --recv-keys 6D398DBD30DD78941E2C4797FE2A5F8BDC282033 && \
+    apt-get update && \
     apt-get install -y \
         gawk \
         wget \