[docs] Update Dockerfile for local dependency

Summary: Neglected to update this when moved chronik-client to local install of ecashaddrjs

Test Plan:
Change line 35 of Dockerfile to read ARG NGINX_CONF=nginx.preview.conf

docker build -t chronik-docs_local .
docker run --rm -p 8080:80 --name chronik-docs chronik-docs_local

Navigate to localhost:8080 and see docs site

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16488