update existing images to alpine 3.15

[tobybellwood]

Quite a bit here - firstly, all images are updated to alpine 3.15 (https://alpinelinux.org/posts/Alpine-3.15.0-released.html)

Secondly, we've added the following images:

• Python 3.10
• Postgres 13 and Postgres 14 (and -drupal variants)
• MariaDB 10.6 (and -drupal variant)
• Solr 8 has been moved from an "Experimental" to a published image

Thirdly, we've added additional testing for the above images