Upgrade focal

[tbille]

Done

• Pin gitdb: see here https://github.com/canonical-web-and-design/docs.ubuntu.com/pull/234
• Upgrade images to focal
• Use build_id instead of revision id which is our pattern

QA

• ./run build
• docker build --build-arg BUILD_ID=build_id -t maas-docs .
• docker run -ti -p "8006:80" --env SECRET_KEY=secret_key maas-docs
• http://localhost:8006/stable/en/
• Play around the site. Should be as usual

[nottrobin]

CI error