Do not bust cached npm-installed dependencies when source files change

Summary:
Similar to D9157, this patch reduces cache busts related to npm packages
and improves rebuild time by ~55% (~70 seconds on my machine).

Depends on D9157

Test Plan:

Run this test plan before and after this patch:

docker build --no-cache -t cashtab .

Change a Cashtab source file, then:

docker build -t cashtab .

Sanity check:

docker run -it -p 9000:80 cashtab

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9158