Fix database credentials when frontend is run with docker

[3wnbr1]

When running VCBD with docker using this sequence :

• ./setup.bash
• ./docker/docker-admin.sh init
• ./docker/docker-admin.sh crawl_patches

Running crawl_patches using docker-admin.sh will fail at some point due to database credentials environment variables not being passed to the frontend container.

sqlalchemy.exc.OperationalError: (MySQLdb._exceptions.OperationalError) (1045, "Access denied for user 'root'@'172.18.0.4' (using password: NO)")

Setting the default environment variable value seems LGTM but there might be better designs to consider.

[evonide]

This looks good as a fix for now thanks!