feat: Make use of rpm-ostree cache (#68)

After setting up the tmpfs mount for /var, rpm-ostree started to not
have cache throughout the single build. This creates a cache for
rpm-ostree that is tied to the specific recipe being built. This will
allow subsequent builds of a recipe to be faster and not interfere with
the cache of another recipe, especially if they are on different OS
versions

templates/Containerfile

@@ -81,6 +81,7 @@ RUN \
   --mount=type=bind,from=stage-modules,src=/modules,dst=/tmp/modules,rw \
       {%- endif %}
   --mount=type=bind,from=stage-exports,src=/exports.sh,dst=/tmp/exports.sh \
+  --mount=type=cache,dst=/var/cache/rpm-ostree,id=rpm-ostree-cache-{{ recipe.name }}-{{ recipe.image_version }},sharing=locked \
   chmod +x /tmp/modules/{{ type }}/{{ type }}.sh \
   && source /tmp/exports.sh && /tmp/modules/{{ type }}/{{ type }}.sh '{{ self::print_module_context(module) }}'
     {%- endif %}