fix(docker): stop COPYing removed alembic_location/alembic_poo into the image

M1-T04 deleted the alembic_location / alembic_poo chains and their .ini files,
but the Dockerfile still COPYed those four paths, so the release image build
failed at 'COPY alembic_poo.ini ./' (path not found). Drop the four stale COPY
lines (only alembic_app remains). Add test_dockerfile_copy_sources_exist, which
asserts every Dockerfile COPY source exists in the build context, so this class
of breakage fails pytest instead of only surfacing in the image CI.

Verified with a real local 'docker build' (succeeds) and pytest (98 passed).

Dockerfile

@@ -11,10 +11,6 @@ RUN pip install --no-cache-dir -r requirements.txt
 COPY app ./app
 COPY alembic_app ./alembic_app
 COPY alembic_app.ini ./
-COPY alembic_location ./alembic_location
-COPY alembic_location.ini ./
-COPY alembic_poo ./alembic_poo
-COPY alembic_poo.ini ./
 COPY scripts ./scripts
 COPY docker ./docker
 COPY README.md ./