ci: Automate container building in Gitlab

This removes the need to manually build and push out the Docker images
for the build. This step is now in GitLab CI itself.

Dockerfile

@@ -1,33 +0,0 @@
-FROM fedora:31
-ARG FEDORA_VER=31
-
-LABEL description="Fedora-based environment for building PipeWire" \
-      maintainer="George Kiagiadakis <george.kiagiadakis@collabora.com>"
-
-# Install pipewire dependencies
-RUN dnf -y install \
-    which \
-    gcc \
-    meson \
-    systemd-devel \
-    dbus-devel \
-    glib-devel \
-    gstreamer1-devel \
-    gstreamer1-plugins-base-devel \
-    jack-audio-connection-kit-devel \
-    pulseaudio-libs-devel \
-    alsa-lib-devel \
-    libv4l-devel \
-    libX11-devel \
-    SDL2-devel \
-    libva-devel \
-    bluez-libs-devel \
-    sbc-devel \
-    doxygen \
-    graphviz \
-    xmltoman \
-    vulkan-loader-devel \
-    git \
-    make \
-    findutils \
-    && dnf clean all