add Dockerfile to build a docker buildserver image

This is just the first step to getting full Docker support for everywhere
the buildserver is used.  This will at least let us replace the hacks that
are separately maintained in fdroid/ci-images-base and
fdroid/ci-images-client.

buildserver/Dockerfile

@@ -0,0 +1,32 @@
+
+FROM debian:stretch
+
+ENV LANG=C.UTF-8 \
+    DEBIAN_FRONTEND=noninteractive
+
+RUN echo Etc/UTC > /etc/timezone \
+	&& echo 'APT::Install-Recommends "0";' \
+		'APT::Install-Suggests "0";' \
+		'APT::Acquire::Retries "20";' \
+		'APT::Get::Assume-Yes "true";' \
+		'Dpkg::Use-Pty "0";' \
+		'quiet "1";' \
+        >> /etc/apt/apt.conf.d/99gitlab
+
+# setup 'vagrant' user for compatibility
+RUN useradd --create-home -s /bin/bash vagrant && echo -n 'vagrant:vagrant' | chpasswd
+
+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 upgrade \
+	&& apt-get dist-upgrade \
+	&& apt-get install sudo \
+	&& apt-get autoremove --purge \
+	&& apt-get clean \
+	&& rm -rf /var/lib/apt/lists/*
+
+# Vagrant sudo setup for compatibility
+RUN echo 'vagrant ALL = NOPASSWD: ALL' > /etc/sudoers.d/vagrant \
+	&& chmod 440 /etc/sudoers.d/vagrant \
+	&& sed -i -e 's/Defaults.*requiretty/#&/' /etc/sudoers