[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