Update app name in 'build docker' step
Update app name in 'build docker' step

When adding the 'build docker image' step in the CircleCi config I
incorrectly '-t pvb-public .', it's actually just looking for '-t app
.'.  The correct docker image/tag combo isn't able to be found and
therefore the workflow fails when trying to push the docker image.

ministryofjustice/prison-visits-public,ministryofjustice/prison-visits-public,ministryofjustice/prison-visits-public,ministryofjustice/prison-visits-public