Do not bust cached apt-install'd dependencies when source files change

Summary:
Copying the source files into the container before installing dependencies with apt-get
forces a cache bust when any Cashtab files change. This saves ~25% (30 seconds) of build time when
rebuilding the docker image.

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

Differential Revision: https://reviews.bitcoinabc.org/D9157