Add Solr 8 images

[tobybellwood]

This PR adds experimental support for Solr 8 to Lagoon.

The images created here are available at testlagoon/solr-8:experimental and testlagoon/solr-8-drupal:experimental.

Please note that they are currently EXPERIMENTAL and, as such, are not suitable for use in production as they may change before they reach GA.

In order to utilise these images, you can either use them

1. "as-is" and reference the image in your docker-compose.yml file - they come with a built-in drupal-compatible core (from search-api-solr)
2. in a dockerfile as the FROM source, so that you can load a custom Drupal configuration in the place of the default one, or load a different config completely.

In the second case, you will need to use a custom command in your CMD like so:

FROM testlagoon/solr-8-drupal:experimental

COPY lagoon/my-solr-config/ /solr-conf/conf

CMD solr-recreate drupal /solr-conf && solr-foreground

the solr-recreate command will ensure that the updated configuration is loaded into the correct location in the volume prior to the solr process starting solr-foreground

In addition, this PR adds the logic to build and publish experimental images outside of the formal tag release process.

[tobybellwood]

Now that these are no longer experimental, replace the above docker images with uselagoon/solr-8:latest or uselagoon/solr-8-drupal:latest (or pin a release version like 22.7.0), but the rest of the dockerfile is correct.