Docker: Update to node:16

[azu]

I think we need to raise the base image to at least node 16.
It will be major release and honkit@4 image use node 16 and drop to support node 12.
It means that If user want to use node 12, the user need to use honkit@3 image.

Originally posted by @azu in #288 (comment)

TODO

• Drop to support Node.js 12

  • honkit/.github/workflows/test.yml

    Lines 13 to 20 in 749bff2

     node-version: [ 12, 14, 16 ] 
     os: [macos-latest, windows-latest, ubuntu-18.04] 
     exclude: 
       # mac tests is expensive, limit to 1 instance 
       - os: macos-latest 
         node-version: 10 
       - os: macos-latest 
         node-version: 12 

  • 14, 16, 18 will test matrix
  • use ubuntu-latest
• update node:12 to node:16 in Dockerfile #289