Add GitLab's CI/CD configuration

The CI/CD pipeline is configured the following way:
 1. Build docker images and add them to repo's registry (manual step for
    now, automation needs GitLab to be updated).
 2. Build igt with meson on Debian and Fedora.
 3. Run `ninja tests` on Fedora.
 4. Build and publish docs as an artifact, for GitLab Pages.

v2: Typos and fully-featured meson build.
v3: Use variables instead of hard-coding registry and project.

Cc: Petri Latvala <petri.latvala@intel.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>