Containerfile: initial version

This commit adds an initial Containerfile to build an `image-builder`
container.

Use all the DNF commands together and add a cleaning command to the end
of it. This makes fewer layers and keeps the image size slightly
smaller.

Co-authored-by: Simon de Vlieger <supakeen@redhat.com>
Signed-off-by: Simon de Vlieger <supakeen@redhat.com>

entrypoint.sh

@@ -0,0 +1,24 @@
+#!/bin/sh
+
+set -e
+
+# TODO: share code with bib to do the setup automatically
+# see https://github.com/teamsbc/container-for-osbuild/blob/main/entrypoint.bash (thanks simon)
+# and https://github.com/osbuild/bootc-image-builder/blob/main/bib/internal/setup/setup.go#L21 (thanks ondrej,achilleas,colin)
+mkdir /run/osbuild
+mkdir /run/osbuild-store
+
+mount -t tmpfs tmpfs /run/osbuild
+mount -t tmpfs tmpfs /run/osbuild-store
+
+cp -p /usr/bin/osbuild /run/osbuild/osbuild
+
+chcon system_u:object_r:root_t:s0 /run/osbuild-store
+chcon system_u:object_r:install_exec_t:s0 /run/osbuild/osbuild
+
+mount -t devtmpfs devtmpfs /dev
+mount --bind /run/osbuild/osbuild /usr/bin/osbuild
+
+# XXX: make this nicer
+cd /output
+/usr/bin/image-builder --store=/store "$@"