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.

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