Containerfile: drop type=cache mount from RUN

We're currently not using this cache /workdir and we ultimately
can't right now until [1] lands in an rpm-ostree release.

See some background context on this in [2] [3].

[1] https://github.com/coreos/rpm-ostree/pull/5391
[2] https://gitlab.com/fedora/bootc/base-images/-/merge_requests/180
[3] https://github.com/coreos/rpm-ostree/pull/5386

Containerfile

@@ -28,8 +28,7 @@ ARG MANIFEST=fedora-standard
 COPY . /src
 WORKDIR /src
 RUN rm -vf /src/*.repo
-RUN --mount=type=cache,target=/workdir \
-    --mount=type=bind,rw,from=repos,src=/,dst=/repos <<EORUN
+RUN --mount=type=bind,rw,from=repos,src=/,dst=/repos <<EORUN
 set -xeuo pipefail
 # Put our manifests into the builder image in the same location they'll be in the
 # final image.