ci: Add .gitlab-ci.yml

Just builds on Fedora 28 for x86_64 at the moment, but it's a start.
Credit to Daniel Stone for eliminating the nested docker image.

Signed-off-by: Adam Jackson <ajax@redhat.com>

.gitlab-ci.yml

+image: fedora:28
+
+job:
+    script:
+    - dnf -y install dnf-plugins-core
+    - dnf -y groupinstall buildsys-build
+    - dnf -y builddep pixman
+    - ./autogen.sh
+    - make -sj4 check
\ No newline at end of file

contrib/ci.sh

+#!/bin/sh
+
+set -ex
+
+./autogen.sh
+make -sj4 check