Containerfile: add temporary hack for --from handling

Using `--from` to set the target releasever doesn't work yet because we
_need_ to set the `releasever` field since the repo files reference it.
There's a pending rpm-ostree PR to fix this but for now hack around it
by manually injecting the necessary variables in the treefile.

Containerfile

@@ -39,6 +39,12 @@ COPY . /src
 WORKDIR /src
 RUN rm -vf /src/*.repo
 COPY --from=repos /etc/yum.repos.d/*.repo /src
+# Brutally inject releasever and repos into manifest file. This is a major hack
+# until rpm-ostree does this on its own: https://github.com/coreos/rpm-ostree/pull/5136
+RUN --mount=type=bind,from=repos,src=/,dst=/repos source /repos/etc/os-release && \
+ echo -e "\nreleasever: $VERSION_ID" >> ${MANIFEST} && \
+ echo -e "\nrepos:\n" >> ${MANIFEST} && \
+ dnf repolist --setopt=reposdir=. | tail -n +2 | cut -f1 -d' ' | sed 's/^/- /' >> ${MANIFEST}
 RUN --mount=type=cache,target=/workdir \
     --mount=type=bind,rw=true,src=.,dst=/buildcontext,bind-propagation=shared \
       rpm-ostree compose image --image-config fedora-bootc-config.json \

README.md

@@ -42,8 +42,6 @@ Fedora repos and dnf variables. E.g.:
 podman build --from quay.io/fedora/fedora:41 ...
 ```
 
-WIP: THIS DOESN'T WORK YET.
-
 ### Deriving
 
 You are of course also free to fork, customize, and build base images yourself.