[Dockerfile] On 16.04

Dear @nim65s, Trying the operation:

docker build -t talos-integration-tests:16.04 --build-arg UBUNTU_VERSION=16.04 --build-arg UBUNTU=xenial --build-arg ROS_DISTRO=kinetic .

I get the following error:

Processing triggers for libc-bin (2.23-0ubuntu11) ...
gpg: unknown option 'import-show'
gpg: invalid import options