Update README.md to fix image build instructions.

[jimmywan]

Fix docker build to use --build-arg.

Checklist

Not all of these might apply to your change but the more you are able to check the easier it will be to get your contribution merged.

• [ ] CI passes
• [x] Description of proposed change
• [x] Documentation (README, docs/, man pages) is updated
• [x] Existing issue is referenced if there is one
• [ ] Unit tests for the proposed change

Fixes #58

[mrtazz]

thanks for the contribution!