dockerfile: Use xenial base image
dockerfile: Use xenial base image

conorsch/securedrop,conorsch/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,conorsch/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,conorsch/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,heartsucker/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,conorsch/securedrop