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).
tliu93
c1a5d7a425