Switch Docker image to use Flathub's flatpak-builder-lint

This way, all tools match the one used on Flathub infra, and reduce
potential breakage caused by Fedora bumping packages.

Dockerfile

@@ -1,13 +1,8 @@
-FROM fedora:latest
+FROM ghcr.io/flathub/flatpak-builder-lint:latest
 
-RUN dnf update -y && \
-    dnf install -y dbus-daemon flatpak flatpak-builder git-lfs python3-aiohttp python3-tenacity python3-gobject xorg-x11-server-Xvfb ccache zstd libappstream-glib && \
-    dnf clean all
+ENTRYPOINT []
+ENV TMPDIR /tmp
 
 RUN flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
 RUN flatpak remote-add --if-not-exists flathub-beta https://flathub.org/beta-repo/flathub-beta.flatpakrepo
 RUN flatpak remote-add --if-not-exists gnome-nightly https://nightly.gnome.org/gnome-nightly.flatpakrepo
-
-RUN flatpak install flathub org.flatpak.Builder -y
-
-ADD --chmod=755 https://raw.githubusercontent.com/flatpak/flat-manager/b07fe8652357ce16d9625f72f9acceaca94d860a/flat-manager-client /usr/bin