Split niri and gnome builds using a matrix workflow, for now.

Containerfile.gnome

@@ -0,0 +1,35 @@
+ARG FEDORA_BASE=43
+
+# Allow build scripts to be referenced without being copied into the final image
+FROM scratch AS ctx
+COPY build_files /
+COPY /sys_files /sys_files
+COPY cosign.pub /cosign.pub
+
+# Base Image
+FROM quay.io/fedora-ostree-desktops/silverblue:$FEDORA_BASE
+
+### [IM]MUTABLE /opt
+## Some bootable images, like Fedora, have /opt symlinked to /var/opt, in order to
+## make it mutable/writable for users. However, some packages write files to this directory,
+## thus its contents might be wiped out when bootc deploys an image, making it troublesome for
+## some packages. Eg, google-chrome, docker-desktop.
+##
+## Uncomment the following line if one desires to make /opt immutable and be able to be used
+## by the package manager.
+
+RUN rm /opt && mkdir /opt
+
+### MODIFICATIONS
+## make modifications desired in your image and install packages by modifying the build.sh script
+## the following RUN directive does all the things required to run "build.sh" as recommended.
+
+RUN --mount=type=bind,from=ctx,source=/,target=/ctx \
+  --mount=type=cache,dst=/var/cache \
+  --mount=type=cache,dst=/var/log \
+  --mount=type=tmpfs,dst=/tmp \
+  /ctx/gnome/build.sh && /ctx/gnome/cleanup.sh
+
+### LINTING
+## Verify final image and contents are correct.
+RUN bootc container lint