https://pastebin.ubuntu.com/p/rhmbMq5npN/

template.json

{
    "builders": [
        {
            "type": "qemu",
            "iso_url": "http://cloud-images.ubuntu.com/releases/bionic/release/ubuntu-18.04-server-cloudimg-amd64.img",
            "iso_checksum_url": "http://cloud-images.ubuntu.com/releases/bionic/release/SHA256SUMS",
            "iso_checksum_type": "sha256",
            "disk_image": true,
            "disk_size": 5120,
            "disk_interface": "virtio-scsi",
            "disk_discard": "unmap",
            "ssh_username": "packer",
            "ssh_password": "packerpassword",
            "http_directory": "cloud-data",
            "qemuargs": [
                ["-smbios", "type=1,serial=ds=nocloud-net;instance-id=packer;seedfrom=http://{{ .HTTPIP }}:{{ .HTTPPort }}/"]
            ],
            "use_default_display": true
        }
    ],
    "provisioners": [
        {
            "type": "shell",
            "inline": ["echo Your steps go here."]
        },

        {
            "type": "shell",
            "execute_command": "sudo sh -c '{{ .Vars }} {{ .Path }}'",
            "remote_folder": "/tmp",
            "inline": [
                "/usr/bin/apt-get clean",
                "rm -r /etc/apparmor.d/cache/* /etc/apparmor.d/cache/.features /etc/netplan/50-cloud-init.yaml /etc/ssh/ssh_host* /etc/sudoers.d/90-cloud-init-users",
                "/usr/bin/truncate --size 0 /etc/machine-id",
                "/usr/bin/gawk -i inplace '/PasswordAuthentication/ { gsub(/yes/, \"no\") }; { print }' /etc/ssh/sshd_config",
                "rm -r /root/.ssh",
                "rm /snap/README",
                "find /usr/share/netplan -name __pycache__ -exec rm -r {} +",
                "rm /var/cache/pollinate/seeded /var/cache/snapd/* /var/cache/motd-news",
                "rm -r /var/lib/cloud /var/lib/dbus/machine-id /var/lib/private /var/lib/systemd/timers /var/lib/systemd/timesync /var/lib/systemd/random-seed",
                "rm /var/lib/ubuntu-release-upgrader/release-upgrade-available",
                "rm /var/lib/update-notifier/fsck-at-reboot /var/lib/update-notifier/hwe-eol",
                "find /var/log -type f -exec rm {} +",
                "rm -r /tmp/* /tmp/.*-unix /var/tmp/*",
                "for i in group gshadow passwd shadow subuid subgid; do mv /etc/$i- /etc/$i; done",
                "rm -r /home/packer",
                "/bin/sync",
                "/sbin/fstrim -v /"
            ]
        }
    ]
}

user-data.yml

#cloud-config
ssh_pwauth: true
users:
  - name: packer
    sudo: ALL=(ALL) NOPASSWD:ALL
    groups: users, admin
    passwd: $6$rounds=4096$XxQGokSw4FI8unZF$lAnQ0ZSMuCvSlv.rFjcxOpyAZr/ZDwtaI/X6BSSH0wtKngvprmgr9nvSMV/dBzE.TJ7Tvd8y0.T50dW5Bi1vf/
    lock_passwd: false
apt:
    preserve_sources_list: true
package_update: false

The pastebin link in the title is from https://multipass.run/docs/building-multipass-images-with-packer, but you had to log in to view it.