docker: use tags

[alexislefebvre]

In the examples, the Docker images don't specify a version:

docker/.examples/apache/docker-compose.yml

Line 5 in 63bc1cf

image: mariadb

If mariadb publish a new release that break backward compatibility (e.g. a new major release), people may have issues when pulling new versions.

I suggest to specify tags like this:

services:
  db:
    image: mariadb:10.7
    # …

  app:
    image: matomo:4.5-apache

So that users will have to update these numbers to get new major releases of these images.