[Cashtab] Update Dockerfile to support ecash-lib as dependency

Summary: Overlooked this in implementing ecash-lib.

Test Plan: docker build -f cashtab.Dockerfile -t cashtab_local .

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16258