[ecash-lib] Dockerfile for CI publication to npm

Summary: Add a top-level Dockerfile to build and deploy ecash-lib to npm on version bumps

Test Plan: docker build -f ecash-lib.Dockerfile -t ecash-lib_local .

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16126