[ecash-wallet] Add Dockerfile for CI deployments

Summary: Now that ecash-wallet is at an MVP state, configure CI publishing

Test Plan:
docker build -f ecash-wallet.Dockerfile -t ecash-wallet_local . builds

I will configure CI in teamcity and need to test there after this lands

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D18383