[chronik] Add a docker image for an agora enabled bitcoind

Summary: This will make it easier to deploy a chronik instance with the plugins enabled.

Test Plan:

cd contrib/docker/chronik-agora
docker build -t agora .
mkdir -p ~/test
docker run -v ~/test:/data agora:latest

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D17487