[ecash-coinselect] Add Dockerfile for running npm publish in ci

Summary:
Looking to get ABC-owned npm modules published in CI when a version number changes.

Potentially we don't need docker for this, but this does allow us to make sure that the server doing the deployment is running node as expected and has npm publish available

Test Plan:
Here is what I did locally, though there could still be some issues that will need to be troubleshooted in CI

docker build -t ecash-coinselect_local .
docker run --rm -p 8080:80 --name ecash-coinselect ecash-coinselect_local

This threw auth errors, expected as the auth token is not available -- but demonstrating that npm publish did run.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14931