Configure Home Assistant Podman container

The default Home Assistant configuration does not allow running a
reverse proxy in front of the application.  To enable this, the
`use_x_forwarded_for` and `trusted_proxies` options have to be set.

Since we want `/var/lib/homeassistant` to be a Btrfs subvolume, we can't
simply include the necessary files in the correct location in the rootfs
image.  Instead, we must define "copy tree" (`C`) actions for
`systemd-tmpfiles` to copy them from `/usr/share/factory`.
Unfortunately, `systemd-tmpfiles` considers `v` and `C` actions
conflicting, and thus will not copy the directory contents recursively.
Each file has to be listed explicitly.

yellow/overlay/etc/containers/systemd/home-assistant.container

@@ -0,0 +1,19 @@
+[Unit]
+After=network-online.target
+Wants=network-online.target
+
+[Container]
+Image=ghcr.io/home-assistant/yellow-homeassistant:stable
+Network=host
+Volume=/var/lib/homeassistant:/config:Z
+User=300
+Group=300
+ReadOnly=true
+VolatileTmp=yes
+
+[Service]
+TimeoutStartSec=5min
+UMask=0077
+
+[Install]
+WantedBy=multi-user.target