Use BASE_URL and not PUBLIC_IPV4 in docker_driver

Using a config intended for an IP address instead of a FQDN here https://github.com/CSC-IT-Center-for-Science/pebbles/blob/master/pebbles/drivers/provisioning/docker_driver.py#L410 causes certificate mismatch issues.

I believe BASE_URL is the one we should be using. Then we need to check that the url is well-formed though so there are no double slashes.

Inspect pebbles/drivers/provisioning/docker_driver.py around line 410, comparing the use of PUBLIC_IPV4 with BASE_URL. Verify how BASE_URL is formed and ensure the resulting URL is well-formed without double slashes. Done means the driver uses the FQDN-based configuration and avoids certificate mismatch issues.

Written by the indexing model from the issue text.