[libxml2] Update GitLab CI container

[Nick Wellnhofer <nwellnhof src gnome org>]

commit af893a58c60d499383d5eec54d6414acd198738c
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Thu Jun 11 16:08:16 2020 +0200

    Update GitLab CI container

 .gitlab-ci.yml | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index de5f4bc6..801d4bcd 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -2,14 +2,16 @@ default:
   # The image was generated with the following Dockerfile. It is also used
   # for libxslt, that's why we need git and libgcrypt-dev.
   #
-  # FROM ubuntu:19.04
+  # FROM ubuntu:20.04
+  # ENV DEBIAN_FRONTEND=noninteractive
   # RUN apt-get update && \
   #     apt-get upgrade -y && \
   #     apt-get install -y --no-install-recommends \
   #         curl git ca-certificates \
   #         autoconf automake libtool pkg-config \
   #         make gcc clang llvm \
-  #         python-dev zlib1g-dev liblzma-dev libgcrypt-dev
+  #         python-dev zlib1g-dev liblzma-dev libgcrypt-dev \
+  #         cmake
   # WORKDIR /tests
   # RUN curl https://www.w3.org/XML/Test/xmlts20080827.tar.gz |tar xz
   image: registry.gitlab.gnome.org/gnome/libxml2