Add mount caching to Dockerfiles

Author: dshemetov

dev/docker/python/Dockerfile

@@ -1,4 +1,4 @@
 # start with the `delphi_python` image
 FROM delphi_python
 
-RUN pip install --no-cache-dir -r repos/delphi/delphi-epidata/requirements.txt -r repos/delphi/delphi-epidata/requirements.dev.txt
+RUN --mount=type=cache,target=/root/.cache/pip pip install -r repos/delphi/delphi-epidata/requirements.txt -r repos/delphi/delphi-epidata/requirements.dev.txt