From man interfaces:

INET6 ADDRESS FAMILY
       This section documents the methods available in the inet6 address family.

   The auto Method

     [...]

       Options

              privext int
                     Privacy extensions (RFC4941) (0=off, 1=assign, 2=prefer)

So I put the following to /etc/interfaces.d/eth0:

iface eth0 inet6 auto
  privext 2