test: local dev container uses local mount for faster testing

[vjeeva]

@jdcallet brought this up as a good feature to have.

This change allows for usage of make local-dev to not require you to clean up your local environment to alter your code and build a new testable copy of belt.

You can now instead run make local-dev to spin up the SRC/DST DB containers, and your belt container instead builds belt based on the local code you have on your laptop as a volume. Therefore, when you make a code change, you can just run pip install -e . to update your test container's belt binary instead of rebuilding everything.

Note: it is indeed redundant to volume while the container is built with the build-time copy of the pgbelt code. I decided against removing that local copy in the Dockerfile to keep our CI steps from not needing to install poetry dependencies on each CI step (see the black and flake8 steps - they are run using that built container).