[ecash-agora] Add a Dockerfile for CI publishing

Summary:
Now that we have ecash-lib and chronik-client both published as working versions using only npmjs dependencies, we can publish ecash-agora the same way.

A working version using this approach is at https://www.npmjs.com/package/ecash-agora/v/0.2.2-rc

ecash-agora is not officially published yet and does not have CI publishing configured yet.

Test Plan: docker build -f ecash-agora.Dockerfile -t ecash-agora_local .

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17243