ci: Use pre-built Docker images for CI builds

Rather than updating the packages on a generic Debian Unstable image
every time a CI build happens, pre-build the image and pre-download all
the dependencies.

This should speed the CI runs up; they currently take about 4 minutes.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

.gitlab-ci/debian-unstable.Dockerfile

@@ -0,0 +1,42 @@
+FROM debian:unstable
+
+RUN apt-get update -qq && apt-get install --no-install-recommends -qq -y \
+    gettext \
+    git \
+    gtk-doc-tools \
+    lcov \
+    libaccountsservice-dev \
+    libappstream-glib-dev \
+    libflatpak-dev \
+    libgirepository1.0-dev \
+    libglib2.0-dev \
+    libglib-testing-0-dev \
+    libgtk-3-dev \
+    libpam0g-dev \
+    libpolkit-gobject-1-dev \
+    libxml2-utils \
+    locales \
+    meson \
+    pkg-config \
+    policykit-1 \
+    python3-pip \
+ && 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
+
+ENV LANG=C.UTF-8 LANGUAGE=C.UTF-8 LC_ALL=C.UTF-8
+
+RUN pip3 install meson==0.54.3
+
+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
+
+COPY cache-subprojects.sh .
+RUN ./cache-subprojects.sh
+
+ENV LANG=C.UTF-8 LANGUAGE=C.UTF-8 LC_ALL=C.UTF-8