[Cashtab] Patch dockerfile to build ecash-agora correctly
Summary: D16496 broke Cashtab deployments as the Dockerfile is missing a COPY statement. Add it here.
Test Plan: docker build -f cashtab.Dockerfile -t cashtab_local .
Reviewers: #bitcoin_abc, emack
Reviewed By: #bitcoin_abc, emack
Subscribers: emack
Differential Revision: https://reviews.bitcoinabc.org/D16510