Add concrete instructions for Raspberry Pi 4 Model B

[yuiseki]

Summary

• This pull request add concrete instructions to building the dev/op env of the UNVT on the Raspberry Pi 4 Model B
• First of all, I have described the way for developer can set up a WiFi access point and in addition to this use Docker on the Raspberry Pi 4 Model B
• If you can use Docker on the Raspberry Pi, you should be able to deploy any application that is prepared in Docker format
• As an example, I have described the instructions to deploy vector-tile-builder on the Raspberry Pi 4 Model B

Future works

• Add more example of the Docker image that makes UNVT as truly portable?
• Describe the instructions for more Raspberry Pi other than Raspberry Pi 4 Model B
• Configure as a wireless network bridge with an additional USB WiFi dongle
  • No more wired connections required
• Configure a mesh network with multiple Raspberry Pi?