fix: clarify ostree container commit must be used (#4)

* fix: clarify ostree container commit must be used after each RUN
* chore: use a bash script and simplify Containerfile

---------

Co-authored-by: Benjamin Sherman <benjamin@holyarmy.org>

build.sh

@@ -0,0 +1,27 @@
+#!/bin/sh
+
+set -ouex pipefail
+
+RELEASE="$(rpm -E %fedora)"
+
+
+### Install packages
+
+# Packages can be installed from any enabled yum repo on the image.
+# RPMfusion repos are available by default in ublue main images
+# List of rpmfusion packages can be found here:
+# https://mirrors.rpmfusion.org/mirrorlist?path=free/fedora/updates/39/x86_64/repoview/index.html&protocol=https&redirect=1
+
+# this installs a package from fedora repos
+rpm-ostree install screen
+
+# this would install a package from rpmfusion
+#RUN rpm-ostree install vlc
+
+
+
+#### Change to System Configuration Files
+
+# this example modifies default timeouts to prevent slow reboots from services that won't stop
+sed -i 's/#DefaultTimeoutStopSec.*/DefaultTimeoutStopSec=15s/' /etc/systemd/user.conf
+sed -i 's/#DefaultTimeoutStopSec.*/DefaultTimeoutStopSec=15s/' /etc/systemd/system.conf