[Cashtab] Dockerfile improvements for faster deployments

Summary:
Now that we are bumping package.json version for every change, we will also force npm ci to reinstall everything even if the dependencies did not change.

Hold the version constant in the docker image. Remove unnecessary dependency updates.

Test Plan: docker build -t cashtab_local .

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15378