hack/Containerfile: New file for local container builds

This is a relatively efficient container-oriented build flow
suitable for local iteration.

Signed-off-by: Colin Walters <walters@verbum.org>

.dockerignore

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 .cosa
+_kola_temp
+docs
+ci
+hack
 target

hack/Containerfile

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+# This container build is just a demo effectively; it shows how one might
+# build bootc in a container flow, using Fedora ELN as the target.
+FROM quay.io/centos-bootc/fedora-bootc:eln as build
+RUN dnf config-manager --set-enabled eln-crb && dnf -y install cargo ostree-devel openssl-devel && dnf clean all
+COPY . /build
+WORKDIR /build
+# See https://www.reddit.com/r/rust/comments/126xeyx/exploring_the_problem_of_faster_cargo_docker/
+# We aren't using the full recommendations there, just the simple bits.
+RUN --mount=type=cache,target=/build/target --mount=type=cache,target=/var/roothome make bin-archive && mkdir -p /out && cp target/bootc.tar.zst /out
+
+FROM quay.io/centos-bootc/fedora-bootc:eln
+COPY --from=build /out/bootc.tar.zst /tmp
+RUN tar -C / --zstd -xvf /tmp/bootc.tar.zst && rm -vf /tmp/*