ci: Base docker image on Debian buster instead of testing

By its nature, testing incurs a risk of breaking something every time
we bump the image.

This requires building wayland-protocols locally, since the package in
buster is too old for current Xwayland.

Acked-by: Olivier Fourdan <ofourdan@redhat.com>

.gitlab-ci.yml

@@ -9,7 +9,7 @@
 # using the same tag.
 variables:
     UPSTREAM_REPO: xorg/xserver
-    FDO_DISTRIBUTION_VERSION: testing-slim
+    FDO_DISTRIBUTION_VERSION: buster-slim
     FDO_DISTRIBUTION_EXEC: 'env FDO_CI_CONCURRENT=${FDO_CI_CONCURRENT} bash .gitlab-ci/debian-install.sh'
     FDO_DISTRIBUTION_TAG: "2020-11-16"
 
@@ -31,7 +31,7 @@ stages:
   # Cancel CI run if a newer commit is pushed to the same branch
   interruptible: true
 
-debian-testing:
+debian-buster:
   extends:
     - .fdo.container-build@debian
     - .ci-run-policy

.gitlab-ci/debian-install.sh

@@ -83,7 +83,6 @@ apt-get install -y \
 	python3-mako \
 	python3-numpy \
 	python3-six \
-	wayland-protocols \
 	x11-xkb-utils \
 	x11proto-dev \
 	xfonts-utils \
@@ -93,6 +92,14 @@ apt-get install -y \
 
 cd /root
 
+# Xwayland requires wayland-protocols >= 1.18, but Debian buster has 1.17 only
+git clone https://gitlab.freedesktop.org/wayland/wayland-protocols.git --depth 1 --branch=1.18
+cd wayland-protocols
+./autogen.sh
+make -j${FDO_CI_CONCURRENT:-4} install
+cd ..
+rm -rf wayland-protocols
+
 git clone https://gitlab.freedesktop.org/mesa/piglit.git --depth 1
 
 git clone https://gitlab.freedesktop.org/xorg/test/xts --depth 1