Allow reproducible dpeloyments for dockerhub images

[christf]

on dockerhub there is :latest and :stable - neither of which support reproducible deployments.

This is a problem because rollbacks do not easily work.

As such, please do tag the images with the version numbers of phpldapadmin so we can use

image: osixia/phpldapadmin:1.2.3

in our docker-compose files.