I am using LXD (“Linux container”) based VM. How do I set an LXD container to start on boot in Linux operating system?

You can always start the container when LXD starts on boot. You need to set boot.autostart to true. You can define the order to start the containers in (starting with highest first) using boot.autostart.priority (default value is 0) option. You can also define the number of seconds to wait after the container started before starting the next one using boot.autostart.delay (default value 0) option.

Syntax

Above discussed keys can be set using the lxc tool with the following syntax:
$ lxc config set {vm-name} {key} {value}
$ lxc config set {vm-name} boot.autostart {true|false}
$ lxc config set {vm-name} boot.autostart.priority integer
$ lxc config set {vm-name} boot.autostart.delay integer

How do I set an LXD container to start on boot in Ubuntu Linux 16.10?

Type the following command:
$ lxc config set {vm-name} boot.autostart true
Set an LXD container name ‘nginx-vm’ to start on boot
$ lxc config set nginx-vm boot.autostart true
You can verify setting using the following syntax:
$ lxc config get {vm-name} boot.autostart
$ lxc config get nginx-vm boot.autostart
Sample outputs:

true

You can the 10 seconds to wait after the container started before starting the next one using the following syntax:
$ lxc config set nginx-vm boot.autostart.delay 10
Finally, define the order to start the containers in by setting with highest value. Make sure db_vm container start first and next start nginx_vm
$ lxc config set db_vm boot.autostart.priority 100
$ lxc config set nginx_vm boot.autostart.priority 99
Use the following bash for loop on Linux to view all values:

#!/bin/bash
echo 'The current values of each vm boot parameters:'
for c in db_vm nginx_vm memcache_vm
do
   echo "*** VM: $c ***"
   for v in boot.autostart boot.autostart.priority boot.autostart.delay 
   do
      echo "Key: $v => $(lxc config get $c $v)"
   done
      echo ""
done

Sample outputs:

#!/bin/bash
x=$(lxc list -c n | awk '{ print $2}'  | sed -e '/^$/d' -e '/^NAME/d')
echo 'The current values of each vm boot parameters:'
for c in $x
do
   echo "*** VM: $c ***"
   for v in boot.autostart boot.autostart.priority boot.autostart.delay 
   do
      echo "Key: $v => $(lxc config get $c $v)"
   done
      echo ""
done