Formatting and documentation updates
Reorganize cpppo/* Docker images
o Collect all update/push behaviour into cpppo/docker/GNUmakefile
o Tidy up Dockerfile documentation and organization
o Put all foundational updated Linux image into cpppo/base
o Correctly EXPOSE EtherNet/IP port 44818
Fix up Vagrant and Docker images
o Default to mirrors.kernel.org for apt packages
o Sync the entire user src/ directory to the VM, instead of just cpppo
o Remove trailing comments from ENTRYPOINT and CMD, which are not parsed
  correctly by Docker