ci: Update the Docker image

Don't use something from ancient times, even if we haven't touched
Clutter in a long while.

.gitlab-ci.yml

@@ -6,7 +6,7 @@ stages:
     - _ccache/
 
 meson-fedora-x86_64:
-  image: registry.gitlab.gnome.org/gnome/clutter/master:v1
+  image: registry.gitlab.gnome.org/gnome/clutter/master:v2
   stage: build
   script:
     - bash -x ./.gitlab-ci/test-docker.sh

.gitlab-ci/Dockerfile

-FROM fedora:28
+FROM fedora:31
 
 RUN dnf -y install \
     adwaita-icon-theme \
@@ -57,7 +57,7 @@ RUN dnf -y install \
     xorg-x11-server-Xvfb \
  && dnf clean all
 
-RUN pip3 install meson
+RUN pip3 install meson==0.49.2
 
 ARG HOST_USER_ID=5555
 ENV HOST_USER_ID ${HOST_USER_ID}