Restrict apt to specific Jessie distro

[Brendan Simon]

Hi,

Is there a way to restrict apt to a **specific release** of Jessie. 
e.g. 8.1, 8.2, 8.3, ... ??

I build root filesystems for embedded systems.  The sources.list is set
to Jessie, but the contents of the generated rootfs can change from one
day to the next if there have been updates.  I want to lock into a
specific release and be sure that the packages wont change if I build
now and 6 or 12 months later.

What's the best way to do this?

Thanks,
Brendan.