maybe it helps using the documentation about installing SW6 within a docker setup: https://docs.shopware.com/en/shopware-platform-dev-en/getting-started/installation-guide#docker-installation-recommended
another way would be to use the CLI (https://github.com/shopware/production#setup-and-install) to install it. The error message does look like it is searching for an existing shopware installation where there is none.