Test registration process

[fschmtt]

Set up a workflow that installs the AppTemplate to a Shopware instance.

[fschmtt]

Alternatively, we could build a Docker image (basically shopware/development:8.0-composer-2 with AppTemplate) in a first job.

In the following job we could then use this image and all the features GitHub workflows offer:

• services (replacing the docker-compose commands)
• strategy.matrix (for testing different versions)