bootc-base-imagectl: support extending package list

The current custom base image flow of rebuilding a "built-in" image with
custom repos and then adding your own content separate is reasonable,
but it would be nice if one could augment the list of packages to
install in that initial build rather than as a separate transaction.

Then, you don't have to cleanup after dnf and `/var` content, re-inject
repo definitions, and refetch repo metadata. It also allows building
container images with additional packages without `dnf` necessarily
being in the package set.

We don't want to leak rpm-ostree implementation details, nor do we want
to invent a new format. So just add support for a `--install` arg and a
generic `--args-file` to pass arguments via a file.

We then generate a new treefile on the fly to extend the `packages`
list.

tests/Containerfile.test-derive

@@ -5,7 +5,10 @@ FROM quay.io/fedora/fedora-bootc:rawhide as repos
 
 # This is intentionally a locally built image
 FROM localhost/fedora-bootc as builder
-RUN --mount=type=bind,from=repos,src=/,dst=/repos,rw /usr/libexec/bootc-base-imagectl build-rootfs --manifest=standard/manifest /repos /target-rootfs
+RUN --mount=type=bind,from=repos,src=/,dst=/repos,rw <<EORUN
+echo -e '--install\nltrace' > args.txt
+/usr/libexec/bootc-base-imagectl --args-file args.txt build-rootfs --manifest=standard/manifest /repos /target-rootfs
+EORUN
 
 # This pulls in the rootfs generated in the previous step
 FROM scratch
@@ -15,6 +18,8 @@ set -xeuo pipefail
 . /usr/lib/os-release
 test "$ID" = fedora
 
+rpm -q ltrace
+
 # And install a package
 dnf -y install strace
 dnf clean all