buildserver: default to HTTPS for apt sources

Debian Docker images will soon default to HTTPS for apt sources, so force
it now:
https://github.com/debuerreotype/docker-debian-artifacts/issues/15

buildserver/Dockerfile

@@ -31,9 +31,13 @@ RUN useradd --create-home -s /bin/bash vagrant && echo -n 'vagrant:vagrant' | ch
 # the provision scripts must be run in the same order as in Vagrantfile
 # - vagrant needs openssh-client iproute2 ssh sudo
 # - ansible needs python3
+# Debian Docker images will soon default to HTTPS for apt sources, so force it.
+# https://github.com/debuerreotype/docker-debian-artifacts/issues/15
 RUN printf "path-exclude=/usr/share/locale/*\npath-exclude=/usr/share/man/*\npath-exclude=/usr/share/doc/*\npath-include=/usr/share/doc/*/copyright\n" >/etc/dpkg/dpkg.cfg.d/01_nodoc \
 	&& mkdir -p /usr/share/man/man1 \
 	&& apt-get update \
+	&& apt-get install apt-transport-https ca-certificates \
+	&& sed -i 's,http:,https:,' /etc/apt/sources.list \
 	&& apt-get upgrade \
 	&& apt-get dist-upgrade \
 	&& apt-get install openssh-client iproute2 python3 openssh-server sudo \