[explorer] Add a Dockerfile for running the explorer

Summary: This will make it easy to do automated deployment.

Test Plan:

cd web/explorer
docker build -t explorer .
docker run -p 3035:3035 explorer

Check the explorer runs on http://localhost:3035
To stop the explorer, use docker stop <container id>

Reviewers: #bitcoin_abc, johnkuney

Reviewed By: johnkuney

Subscribers: johnkuney

Differential Revision: https://reviews.bitcoinabc.org/D12978