Played it for the first time last night on the rock that was my IS-85 with Improvised Armour :)
It also had defensive driver.

Defensive Driver
"When rolling defence dice you may re-roll one die"
Clearly from the Def Driver card I got to re-roll one die each time I rolled defence dice.

No Retreat
"When attacking or defending..... you may re-roll one of your dice"
For No Retreat I played it as each time I was defending I got to re-roll one die.
So when I was attacked multiple times by 2 Achilles and a Comet at close range in a most futile attempt to kill the irresistible force I got to re-roll one die for each attack.

Both cards cost the same but there is a restriction on NR that you can only use the re-rolls when no friendly is in close range.

My question is - did I play the doctrine correctly re-rolling for EACH defence roll or is it supposed to be one die regardless of how many attacks I withstand?

I'd say yes. As long as you are not rerolling a die that has already been rerolled *noticeable facial tic*, the effect happens whenever it's qualifier is met. On this case, when it attacks or when it defends. The only ambiguity I'd worry about is whether they mean for you to be able to do both on the same turn or not, because who knows?

As I understand it, you would get the re-roll once for each attack made against that tank.