Use the User ID in the Dockerfile instead of the Username

There is a big trend towards using non-root Docker images. Luckily for us, Solr already uses a non-root user in its Docker image, solr:solr (8983:8983). However when specifying the "USER" at the end of the Dockerfile, it uses in the name instead of the ID. Therefore when certain security features are enabled, such as in Kubernetes, these tools cannot determine if the user "SOLR" is root or non-root. If we change to using the User ID, then these tools will begin to work.