CI: add basic gitlab-ci

This is largely based off of speex's.
8 jobs for feature/ci in 2 minutes (queued for 119 minutes and 18 seconds)
Status Job ID Name Coverage
  Build
passed #10807
docker
autoconf

00:01:12

passed #10808
docker
fixed-point

00:00:50

passed #10809
docker
no-examples

00:00:44

passed #10810
docker
no-float

00:00:46

failed #10803
docker
autoconf

failed #10804
docker
fixed-point

failed #10806
docker
no-examples

failed #10805
docker
no-float