Add workflow for end-to-end test (using Docker)

[hannahbast]

This workflow performs the following end-to-end test using the Docker setup:

1. Check out the code
2. Build a Docker image using the Dockerfile provided by the code
3. Download the PBF for Malta and built a TTL for it
4. Build a QLever index from the TTL, start a server, and query for the number of geometries

[patrickbr]

Thanks, @hannahbast ! Looking over the Dockerfile, we did not fully understand the additional benefit of spinning up a QLever instance and filling it with the generated TTL. As we understand it, this mainly tests (1) if the generated TTL is valid, and (2) whether a roughly expected number of geometries has been written. But isn't (1) already performed in the Build TTL for Malta and check its validity step, and couldn't (2) be achieved by a simple grep | wc -l, without adding the additional QLever dependency?