Can I play Reaction Speed spells before attack and defend triggers? Such as casting Discipline on Ezreal Dashing before his attack trigger occurs?

No, you cannot play Discipline before the attack trigger occurs.

According to RiftJudge FAQ #2235, there is no priority window to play reaction spells before a showdown begins if there are no triggers. As soon as the move to the battlefield is complete, combat/showdown begins immediately, and the "When I attack" triggers are placed on the chain first.

How to play it correctly

You must wait until Ezreal's "When I attack" ability triggers and is placed on the chain. Once that trigger is on the chain, you have a priority window to respond with Discipline.

The correct sequence is:

1. Move: Ezreal moves to an occupied battlefield.
2. Triggers: The "When I attack" triggered ability is placed on the chain.
3. Priority Window: Because a trigger has been placed on the chain, the state is now CLOSED, and players can play [Reaction] spells.
4. Respond: You can now play Discipline in response to the attack trigger. It is placed on top of the chain.
5. Resolution (LIFO):
   • Discipline resolves first, giving Ezreal +2 Might.
   • Ezreal's "When I attack" ability then resolves, checking his new, higher Might value to deal damage.

This interaction is also supported by RiftJudge FAQ #9638, which confirms that you can buff Ezreal in response to his own attack trigger to increase the damage dealt by that ability.