Add Docker icon and GitHub Action workflow

[lyuwen]

The following updates are introduced in the pull request

1. Add Docker icon.
2. Add a GitHub Action script that builds the fonts and CSS files automatically on push and pack and upload them to release asset when a release is created.
3. Update .fontcustom-manifest.json file to match the codepoint to the unicode in the icons.yml file.

[lyuwen]

Actually, I have a question about the .fontcustom-manifest.json file. I'm currently manually editing the file. Is it automatically generated? The fontcustom code tried to generate one itself, but it's Unicode codes of the icons do not match the ones specified in the icons.yml file. So I'm a bit confused.