devcontainer improvements

[gilesknap]

• no longer use host network for dev-containers
• use a shared network called channel_access
• use the host user for docker dev-containers
• supply a compose folder that launches:
  • cagateway to serve PVs from channel access container network to loopback
  • phoebus that loads default index.bob and looks for pvs on loopback

All works for docker and podman.

[GDYendell]

Does this mean the PVs can't be seen on other hosts?