Docker deployment files and instructions

Summary: Add docker deployment files for cashtab

Test Plan:

cd web/cashtab
npm install --legacy-peer-deps
docker-compose build
docker-compose up

Navigate to localhost:8080 to view app

Reviewers: jasonbcox, deadalnix, alcipir, #bitcoin_abc

Reviewed By: alcipir

Differential Revision: https://reviews.bitcoinabc.org/D8573