[CI] Fix the chronik-plugins docker image

Summary: The agora.py file is not available in the destination image and need to be manually copied to the datadir. This is not the intent of this container, and this diff fixes this.

Test Plan:

cd contrib/docker/chronik-plugins
docker build -t chronik-plugins .
docker run chronik-plugins

Check it runs with agora enabled.

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D17820