[e.cash] Add a Dockerfile to deploy the website

Summary: As per title. This prevent version conflicts and will enable previews on CI.

Test Plan:

cd web/e.cash
docker build -t ecash .
docker run -it --rm -p 3000:3000 ecash

Navigate to http://localhost:3000 and enjoy.

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D13650