Install docker on arm64 synology

get-docker.sh

#!/bin/bash
set -e

ARCH=aarch64
DOCKER_VERSION=20.10.9
COMPOSE_VERSION=2.5.1
DOCKER_DIR=/volume1/@docker

echo "Downloading docker $DOCKER_VERSION-$ARCH"
curl "https://download.docker.com/linux/static/stable/$ARCH/docker-$DOCKER_VERSION.tgz" | tar -xz -C /usr/local/bin --strip-components=1

echo "Creating docker working directory $DOCKER_DIR"
mkdir -p "$DOCKER_DIR"

echo "Creating docker.json config file"
mkdir -p /usr/local/etc/docker
cat <<EOT > /usr/local/etc/docker/docker.json
{
  "storage-driver": "vfs",
  "iptables": false,
  "bridge": "none",
  "data-root": "$DOCKER_DIR"
}
EOT

echo "Creating docker startup script"
cat <<'EOT' > /usr/local/etc/rc.d/docker.sh
#!/bin/sh
# Start docker daemon

NAME=dockerd
PIDFILE=/var/run/$NAME.pid
DAEMON_ARGS="--config-file=/usr/local/etc/docker/docker.json --pidfile=$PIDFILE"

case "$1" in
    start)
        echo "Starting docker daemon"
        # ulimit -n 4096  # needed for influxdb (uncomment if your limit is lower)
        /usr/local/bin/dockerd $DAEMON_ARGS &
        ;;
    stop)
        echo "Stopping docker daemon"
        kill $(cat $PIDFILE)
        ;;
    *)
        echo "Usage: "$1" {start|stop}"
        exit 1
esac
exit 0
EOT

chmod 755 /usr/local/etc/rc.d/docker.sh

echo "Creating docker group"
egrep -q docker /etc/group || synogroup --add docker root

echo "Installing docker compose $COMPOSE_VERSION"
curl -SL "https://github.com/docker/compose/releases/download/v$COMPOSE_VERSION/docker-compose-linux-$ARCH" \
     --create-dirs -o /usr/local/lib/docker/cli-plugins/docker-compose
chmod +x /usr/local/lib/docker/cli-plugins/docker-compose
chgrp -R docker /usr/local/lib/docker

echo "Starting docker"
/usr/local/etc/rc.d/docker.sh start

echo "Done.  Please add your user to the docker group in the Synology GUI and reboot your NAS."

I have a DS418 and this script worked, in that the components were installed as expected and docker could be started and stopped.

However, these lines:
https://gist.github.com/DaveMDS/c35d77e51e0186a4fe2e577f51a5b09a#file-get-docker-sh-L20-L21
mean that containers are isolated from the outside world.

If you remove "bridge" and "iptables" from docker.json, then you get error message about "can't initialize iptables table 'nat'" at docker startup. I resolved these by running "modprobe nf_nat_ipv4 iptable_nat" before starting docker. After that, containers are able to make external connections via the bridge network.

Perhaps you could add that to your startup script.