container: add password and wheel group membership

Containerfile

@@ -5,7 +5,9 @@ RUN dnf install -y kernel-devel-$(uname -r | awk -F'-' '{print $1}')
 RUN dnf install -y https://zfsonlinux.org/fedora/zfs-release-3-1$(rpm --eval "%{dist}").noarch.rpm
 RUN dnf install -y zfs
 
-RUN useradd -m -d /var/home/backup backup
+RUN useradd -m -d /var/home/backup -G wheel backup
+ARG PASSWORD
+RUN echo "backup:${PASSWORD}" | chpasswd
 RUN mkdir /backups && chown backup:backup /backups
 
 COPY --chown=root:root --chmod=600 id_ed25519.pub /root/.ssh/authorized_keys