CI: add basic gitlab-ci

This is largely based off of speex's.
4 jobs for master in 1 minute and 52 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build
passed #10817
docker
autoconf

00:01:07

passed #10818
docker
fixed-point

00:00:48

passed #10820
docker
no-examples

00:00:43

passed #10819
docker
no-float

00:00:46