[chronik] Specify the base image version via a build arg for the chronik-agora image

Summary:
So we can have the CI deploy the correct dependency, and use the latest as default.

Depends on D17491.

Test Plan:

docker build --build-arg VERSION=0.30.8 -t agora .

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D17492