Docker mode: Python node images fail to build ("/requirements.txt": not found) #646

In docker mode, mkconcore builds each Python node in its own docker-<node>/ directory that contains the Dockerfile, the node script, concore.py, concore_base.py, the port files and the node's .dir contents. Dockerfile.py starts with COPY requirements.txt ., but that file is only there if the node's .dir provides one, so the build stops:

#7 [4/6] COPY requirements.txt .
ERROR: failed to compute cache key: ... "/requirements.txt": not found

Steps (dev @ 8f3e444, Docker 29.4):

1. python mkconcore.py demo/sample.graphml demo out docker
2. cd out && ./build → both images fail at COPY requirements.txt .

CI doesn't catch this because the docker-build job builds Dockerfile.py from the repo root, where requirements.txt exists.

I have a small fix to Dockerfile.py and will open a PR against dev.