Update Docker base image from 5.0.0 to 6.0.0
Update Docker base image from 5.0.0 to 6.0.0

digitalmarketplace/base-frontend:6.0.0 uses npm for building instead of
yarn.

alphagov/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend