Properly source run-initdb in docker commands

Currently in various docker commands, such as solr-precreate, solr-foreground, solr-create and solr-demo, run-initdb is run as a child process. This means that any environment variables that are exported in run-initdb are ignored after the child-process is complete. We instead need to go back to sourcing run-initdb, so that the environment variables are passed to solr-fg.

This is not the case in the 8.x docker-solr docker images, just the unreleased 9.0 docker image so far.