ci: Add a Debian-based image

We want to test GTK3 on Debian as well.

.gitlab-ci/debian-gtk3.Dockerfile

@@ -0,0 +1,71 @@
+FROM debian:buster
+
+RUN apt-get update -qq && apt-get install --no-install-recommends -qq -y \
+    adwaita-icon-theme \
+    ccache \
+    dconf-gsettings-backend \
+    g++ \
+    gcc \
+    gettext \
+    git \
+    gobject-introspection \
+    gvfs \
+    hicolor-icon-theme \
+    itstool \
+    libatk-bridge2.0-dev \
+    libatk1.0-dev \
+    libc6-dev \
+    libcairo2-dev \
+    libcairo-gobject2 \
+    libcolord-dev \
+    libcups2-dev \
+    libegl1-mesa-dev \
+    libepoxy-dev \
+    libfontconfig1-dev \
+    libfreetype6-dev \
+    libgdk-pixbuf2.0-dev \
+    libgirepository1.0-dev \
+    libglib2.0-dev \
+    libharfbuzz-dev \
+    libjson-glib-dev \
+    libpango1.0-dev \
+    librest-dev \
+    librsvg2-common \
+    libsoup2.4-dev \
+    libwayland-dev \
+    libx11-dev \
+    libxcomposite-dev \
+    libxcursor-dev \
+    libxdamage-dev \
+    libxext-dev \
+    libxfixes-dev \
+    libxi-dev \
+    libxinerama-dev \
+    libxkbcommon-dev \
+    libxkbcommon-x11-dev \
+    libxml2-dev \
+    libxrandr-dev \
+    locales \
+    ninja-build \
+    pkg-config \
+    python3 \
+    python3-pip \
+    python3-setuptools \
+    python3-wheel \
+    shared-mime-info \
+    wayland-protocols \
+    xauth \
+    xvfb \
+ && rm -rf /usr/share/doc/* /usr/share/man/*
+
+# Locale for our build
+RUN locale-gen C.UTF-8 && /usr/sbin/update-locale LANG=C.UTF-8
+
+ARG HOST_USER_ID=5555
+ENV HOST_USER_ID ${HOST_USER_ID}
+RUN useradd -u $HOST_USER_ID -ms /bin/bash user
+
+USER user
+WORKDIR /home/user
+
+ENV LANG=C.UTF-8 LANGUAGE=C.UTF-8 LC_ALL=C.UTF-8