Merge pull request #20092 from jtattermusch/python_fix_docker_build

templates/tools/dockerfile/debian_testing_repo.include

...rfile/interoptest/grpc_interop_python/Dockerfile.template

templates/tools/dockerfile/python_stretch.include

...ockerfile/test/python_stretch_3.6_x64/Dockerfile.template

...ockerfile/test/python_stretch_3.7_x64/Dockerfile.template

...ockerfile/test/python_stretch_3.8_x64/Dockerfile.template

templates/tools/dockerfile/test/sanity/Dockerfile.template

tools/dockerfile/interoptest/grpc_interop_python/Dockerfile

tools/dockerfile/test/python_stretch_2.7_x64/Dockerfile

tools/dockerfile/test/python_stretch_3.5_x64/Dockerfile

tools/dockerfile/test/python_stretch_3.6_x64/Dockerfile

tools/dockerfile/test/python_stretch_3.7_x64/Dockerfile

tools/dockerfile/test/python_stretch_3.8_x64/Dockerfile

tools/dockerfile/test/python_stretch_3.8_x64/get_cpython.sh

tools/dockerfile/test/sanity/Dockerfile